Very little is known of the pre-Christian religion of the early English ('Anglo-Saxons'), because all the surviving Old English texts were written down after the conversion to Christianity and no written account of the previous religious beliefs survives. There are some snippets in Bede's On the Reckoning of Time, some place names, bits of word etymology, fragments in poetry that might be echoes of an older tradition, occasional archaeological finds, and extrapolation from accounts of related cultures such as Tacitus' Germania and the Norse myths. I need hardly say that this is not as firm a basis as one would like for trying to reconstruct a lost religion (!). Nevertheless, it's better than nothing, so with that caveat in mind let's see where we get.
The modern English word "hell", meaning the dwelling-place of the dead, the underworld and/or a place of punishment after death, derives directly from its Old English counterpart "helle".
In King Alfred's translation of Boethius (ninth century), Cerberus, the dog who guards the gates of Hades in Greek and Roman mythology, is called "helle hund".
In the Old English poem Beowulf, the monster Grendel is described as "feond on helle", "an enemy from hell".
According to the Oxford English Dictionary, "helle" is cognate with Old Frisian (helle), Old Saxon (hellia, hel), Old High German (hella), Old Icelandic (hel), and Gothic (halja), probably originally meaning a hole or place of concealment. So the word is widespread in the Germanic languages, and was in use by at least the ninth century. It was probably in use much earlier, since it occurs in several languages and may therefore derive from a time before the languages became differentiated, though it's always possible that the languages borrowed it from each other.
Since "helle hund" was used in relation to Cerberus, "hell" was presumably considered to be roughly equivalent to Hades and was not confined to the Christian concept of hell. No description of the pagan English concept of hell has come down to us, but since the word was cognate with the Old Icelandic Hel, it's a reasonable inference that the concept attached to the word was also similar to the Norse concept. Luckily, we have an idea what that was.
In Snorri Sturluson's Prose Edda, written in Iceland in the thirteenth century, Hel referred both to the goddess of the underworld and to her realm. (This is similar to Greek and Roman mythology, in which Hades referred both to the god of the underworld and to the underworld itself). Snorri gives a vivid description of Hel and her realm:
But evil men go to Hel and thence down to Niflhel [Dark Hel]; and that is down in the ninth world.
--Gylfaginning chapter 3. Prose Edda.
Hel he threw down into Niflheim, and made her ruler over nine worlds. She has the power to dole out lodgings and provisions to those who are sent to her, and they are the people who have died of disease or old age. She has there an enormous dwelling with walls of immense height and huge gates. Her hall is called Eljudnir (Sprayed with Snowstorms), her dish is Hunger, her knife is Famine, her slave is Lazy, and her woman servant is Slothful. The threshold over which people enter is called Fallandaforad (Falling to Peril), her bed is named Kor (Sick-bed) and her bed curtains are called Blikjandabol (Gleaming Disaster). She is half black and half a lighter flesh-colour and is easily recognised). Mostly she is gloomy and cruel.
--Gylfaginning, chapter 34. Prose Edda.
When the Norse god Odin journeys to the realm of Hel to ask questions of a long-dead seeress, she tells him:
I was snowed on with snow, and smitten with rain,
And drenched with dew; long was I dead.
So the Norse Hel was thought of as a miserable place of cold and wet and hunger, presided over by a hideous monster. This is consistent with the description of Grendel's bleak abode in Beowulf:
The fell and fen his fastness was
The march his haunt
--Beowulf, lines 102-103
. walked nightlong
The misty moorland
--Beowulf, lines 161-162
up steep screes, by scant tracks
Where only one might walk, by wall-faced cliffs,
Through haunted fens - uninhabitable country
--Beowulf, lines 1410-1411
Grendel, together with giants, elves and evil spirits, is described in Beowulf as the descendant of Cain, banished to the wastelands by God. Leaving aside the Christian gloss, the picture of a cold, wet, bleak and thoroughly miserable wilderness inhabited by monsters (one of whom, Grendel's mother, is female), is entirely consistent with the Norse description of Hel in the Prose Edda.
So, it seems reasonable to infer that before they converted to Christianity the pagan English had a concept of a cold and miserable place called hell. As the word continued in use after conversion to Christianity as the name for a place of punishment after death, it seems likely that the original concept also included the idea that hell was the afterlife for people who weren't favoured. Whether everyone who died a natural death went there, as Snorri says in Gylfaginning chapter 34, or whether evil people went there, as Snorri says in Gylfaginning chapter 3, is not clear. Quite possibly there were different traditions among different groups of people. If the word originated from a root meaning "hole" as the Oxford English Dictionary says (and I would take their word for most things on word origins), it seems likely that it derived from a description of the grave - a cold, wet, miserable hole in the ground where one went after death in the literal as well as metaphorical sense. This may tie in to the variable funeral customs observed in 'Anglo-Saxon' cemeteries, and I'll come back to this in a later article.
Was "hell" in Old English also used in its other common modern sense, as an expletive and an intensifier in colloquial phrases (What the hell, how the hell, go to hell, hell of a etc)? I have no idea. Formal court poetry doesn't generally use colloquialisms, and Old English poetry is more formal than most because of the demands of the alliterative measure. If there was an Old English dictionary of slang and swearing it certainly hasn't come down to us. Since the word was in use and represented a place that you wouldn't look forward to going to, as in its modern sense, it seems not unreasonable that it might also have been in use as an imprecation, and I use it in this sense in Paths of Exile. However, I think we can reasonably say that phrases that rely on hell being a hot place (when hell freezes over, snowball's chance in hell, a cold day in hell, hell-fire) probably came into use later, after the shift to the Christian concept of hell as a fiery place.
Balder's Dream, available
Beowulf. Translated by Michael Alexander. Penguin Classics, 1973. ISBN 0-14-044268-5.
Oxford English Dictionary. Available online by subscription at www.oed.com
Prose Edda. Translated by Jesse Byock. Penguin Classics, 2005. ISBN 978-0-14-044755-2.