英语词源字典 - 在线词源网站

hair [hair 词源字典]: hair: [OE] No general Indo-European term for ‘hair’ has come down to us. All the ‘hair’-words in modern European languages are descended from terms for particular types of hair – hair on the head, hair on other parts of the body, animal hair – or for single hairs or hair collectively, and indeed many retain these specialized meanings: French cheveu, for instance, means ‘hair of the head’, whereas poil denotes ‘body hair’ or ‘animal hair’.

In the case of English hair, unfortunately, it is not clear which of these categories originally applied, although some have suggested a connection with Lithuanian serys ‘brush’, which might indicate that the prehistoric ancestor of hair was a ‘bristly’ word. The furthest back in time we can trace it is to West and North Germanic *khǣram, source also of German, Dutch, and Danish haar and Swedish hår.

The slang use of hairy for ‘difficult’ is first recorded in the mid 19th century, in an erudite context that suggests that it may have been inspired by Latin horridus (source of English horrid), which originally meant (of hair) ‘standing on end’. Its current use, in which ‘difficult’ passes into ‘dangerous’, seems to have emerged in the 1960s, and was presumably based on hair-raising, which dates from around 1900.

It is fascinatingly foreshadowed by harsh, which is a derivative of hair and originally meant ‘hairy’.
[hair etymology, hair origin, 英语词源]
fascinating (adj.): "bewitching, charming," 1640s, present-participle adjective from fascinate). Related: Fascinatingly.