Is it ensure, insure, or assure?
"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.
A dry, indehiscent simple fruit consisting of one seed surrounded by a hard and thick pericarp (fruit wall). The seed does not adhere to the pericarp but is connected to it by the funiculus. A nut is similar to an achene but larger. Acorns, beechnuts, chestnuts, and hazelnuts are true nuts. Informally, other edible seeds or dry fruits enclosed in a hard or leathery shell are also called nuts, though they are not true nuts. For instance, an almond kernel is actually the seed of a drupe. Its familiar whitish shell is an endocarp found within the greenish fruit of the almond tree. Peanuts are actually individual seeds from a seed pod called a legume.
[insanity sense probably fr late 1800s off one's nut, that is, head; senses 4, 5, and 6 fr the custom of taking the retaining nut from the wheel of a circus wagon, to be returned when all bills were paid]