tough nut to crack

"hard seed," Old English hnutu, from Proto-Germanic *khnut- (cf. Old Norse hnot, Dutch noot, Old High German hnuz, German nuß "nut"), from PIE *kneu- "nut" (cf. Latin nux; see nucleus). Sense of "testicle" is attested from 1915. Nut-brown is from c.1300 of animals; c.1500 of complexions of women.

Meaning "crazy person, crank" is attested from 1903, (British form nutter first attested 1958; nut-case is from 1959); see nuts. American English slang sense of "amount of money required for something" is first recorded 1912. The nut that goes onto a bolt is first recorded 1610s (used of other small mechanical pieces since early 15c.). Nuts and bolts "fundamentals" is from 1960.