Origins
The word 'hair' is one of the oldest body-part terms in English, descending from Old English 'hΗ£r,' from Proto-Germanic *hΔrΔ , from PIE *keres- (bristly, rough, standing on end).βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ The PIE root reveals the original concept behind the word: hair was named not for its texture, color, or beauty, but for its capacity to stand erect β to bristle. This is the same quality that connects 'hair' to one of its most surprising distant relatives: 'horror.'
Latin 'horrΔre' (to bristle, to stand on end, to shudder) derives from the same PIE root, and its meaning makes the connection vivid. When Romans said 'horrΔre,' they meant the physical sensation of hair standing up β the piloerection that accompanies fear, cold, or awe. From this came 'horror' (the bristling feeling), 'horrid' (causing bristling), and 'horrible.' English 'hair' and English 'horror' are thus two reflexes of the same ancient observation: hair is the thing that bristles, and horror is the feeling when it does.
In Old English, 'hΗ£r' was primarily a collective noun referring to the mass of hair on the head, not to individual strands. A single hair or a lock was called a 'locc' (which survives as Modern English 'lock,' as in 'a lock of hair'). This collective sense explains why 'hair' has no regular plural in standard English β we say 'her hair is beautiful,' not 'her hairs are beautiful.' The count-noun usage ('I found a hair in my soup,' 'three hairs on his chin') developed later and still feels slightly marked.
Proto-Indo-European Roots
The Germanic cognates are straightforward: German 'Haar,' Dutch 'haar,' Old Norse 'hΓ‘r,' Old Frisian 'hΔr.' The word is pan-Germanic and well-attested. Outside Germanic, the cognates are less certain, but the connection to Latin 'horrΔre' through PIE *keres- is widely accepted.
The cultural and symbolic weight of hair across human societies is immense β from Samson's strength residing in his hair, to the tonsure of monks, to the elaborate wigs of the eighteenth century, to the political charge of natural Black hair in the twentieth and twenty-first centuries. Linguistically, hair has generated a rich vocabulary: 'hairbreadth' (an extremely small distance, from the width of a hair), 'harebrained' (originally 'hair-brained,' confused with the hare), 'hairpin turn,' and 'hair-trigger.' The word 'mohair' appears to be related but is actually from Arabic 'mukhayyar' (selected, choice fabric), reanalyzed in English as if it contained 'hair.'
The phrase 'let your hair down' (to relax, to be informal) dates from the seventeenth century, when respectable women wore their hair pinned up in public and only released it in private. 'To split hairs' (to make excessively fine distinctions) dates from the seventeenth century as well. 'By a hair' or 'by a hair's breadth' (by the smallest possible margin) goes back to the sixteenth century β the width of a single hair being the smallest unit of measurement the everyday speaker could imagine.