Green’s Dictionary of Slang

fruit loop n.2

[pun on the US breakfast cereal Fruit Loops/loopy adj. (1)]

(US) a crazy or stupid person.

[US]Athens (GA) Banner-Herald/Daily News 8 Aug. 8: It’s sort of a national Fruit Loops day. Lots of windchimes.
[US]P. Cornwell Body of Evidence (1992) 49: You got any idea how many frootloops call in this kind of crap on a regular basis?
[US]L. Fugett Butcher Holler 57: My staying doesn’t appear to be a matter of choice, not according to that skinny fruit-loop you bribed this morning.
[Aus]L. Redhead Cherry Pie [ebook] For all I knew she was a total fucking fruit loop.