1. slang To lose one's reason or sanity; to become crazy or psychotic. In this phrase, "nut" is a derogatory slang term meaning "a crazy person." All those days of sleep deprivation finally caused him to start nutting up. It seems that the former employee nutted up after being fired and began planning an attack against his ex-employers.

2. vulgar slang To start acting in a strong, confident, and/or courageous manner, especially after having previously failed to do so. In this phrase, "nut" is the singular of "nuts," a vulgar slang term for testicles also used figuratively to mean confidence, courage, bravado, etc. The phrase is not exclusively applied to males. Often used as an imperative. You need to nut up and ask your boss for a raise already! Janet, I know you're nervous about asking Tom out on a date, but just nut up and give it a shot!