Origins
The English word 'thousand' descends from Old English 'þūsend,' from Proto-Germanic *þūsundī. Unlike the numerals one through ten and hundred, which have transparent cognates across the entire Indo-European family, 'thousand' is exclusively Germanic — Latin 'mīlle,' Greek 'khī́lioi,' Sanskrit 'sahásra,' and the words for 'thousand' in other IE branches are all unrelated to each other and to the Germanic form. This suggests that a common PIE word for 'thousand' either did not exist or was replaced independently in each branch, perhaps reflecting a stage of culture where quantities above a hundred were rarely needed.
The etymology of Proto-Germanic *þūsundī has been debated for over a century, but the most widely accepted analysis derives it from a compound *tuHs-ḱm̥t-ih₂, built from two PIE elements: *teuH- (to swell, to become strong or numerous — the source of English 'thumb,' originally 'the swollen finger,' and Latin 'tumēre,' to swell, whence English 'tumor') and *ḱm̥tóm (hundred). On this analysis, 'thousand' literally means 'the swelling hundred,' 'the powerful hundred,' or perhaps 'the great multitude of hundreds' — a vivid metaphor for a quantity that seemed vastly, almost immeasurably larger than a hundred to pre-literate Germanic speakers.
The sound changes from the proposed PIE compound to Proto-Germanic *þūsundī are largely regular. PIE *t became Proto-Germanic *þ (by Grimm's Law), PIE *ḱ became *h (also by Grimm's Law), and the compound underwent the kinds of vowel reduction and restructuring typical of long compound words. The final *-ī is a feminine abstract noun suffix, suggesting that 'thousand' was originally conceived as a noun ('a thousand-ness' or 'a great-hundred amount') rather than an adjective modifying a noun.
Germanic Development
Within the Germanic family, the word is remarkably uniform: German 'tausend,' Dutch 'duizend,' Old Norse 'þúsund,' Gothic 'þūsundi,' Old Saxon 'thūsundig,' and Old Frisian 'thūsend' all clearly derive from the same Proto-Germanic form. The phonological differences between them (German /t/ vs. English /θ/, Dutch /d/ vs. Gothic /þ/) reflect the regular sound correspondences between the Germanic daughter languages.
The non-Germanic words for 'thousand' tell their own interesting stories. Latin 'mīlle' is of uncertain etymology, possibly pre-Indo-European. Greek 'khī́lioi' is also opaque, potentially connected to a word for 'multitude.' Sanskrit 'sahásra' may be related to 'sahas' (strength, power), yielding a meaning parallel to the Germanic 'swelling hundred' — 'the powerful number' — though through entirely different vocabulary. The fact that each major IE branch coined its own word for 'thousand' independently is itself significant: it suggests that the PIE-speaking community had little need for numbers above one hundred, consistent with a relatively small-scale pastoral society.
In Old English, 'þūsend' was grammatically a noun, not an adjective. It took a genitive complement: 'þūsend manna' (a thousand of men), not *'þūsend men.' This construction survives in fossilized form in expressions like 'thousands of people,' where 'thousand' behaves as a noun taking 'of' + noun. When used with a specific number ('three thousand soldiers'), it has been reanalyzed as a numeral modifier, but the noun origin is still visible in the plural form 'thousands.'
Latin Roots
Culturally, 'thousand' has long served as a round number signifying 'very many' — the medieval term 'chiliad' (from Greek 'khī́lioi,' thousand) was used to discuss Christ's thousand-year reign (the 'millennium,' from Latin 'mīlle'), and the year 1000 CE generated widespread apocalyptic anxiety in Christendom. The word 'millennium' itself combines Latin 'mīlle' (thousand) with 'annus' (year), and its adoption into English in the 17th century created a learned synonym for what Germanic 'thousand-year' would express natively.