Answers:
there are two supposed origins -
1. is from the word nut, meaning hard seed, which is called "hnutu" in Old English or "*khnut" in Proto-Germanic
2. from earlier be nutts upon "be very fond of" (1785), which is possibly from nuts (noun, plural) "any source of pleasure" (1617), from nut (described above). Sense influenced probably by metaphoric application of nut to "head" (1846, e.g. to be off one's nut "be insane," 1860). Nut "crazy person, crank" is attested from 1903, (British form nutter first attested 1958). Source(s):
what is language game?