Green’s Dictionary of Slang

lunch v.2

[out to lunch adj.]

1. (US campus) to lack concentration, to be unaware.

[US]Eble Campus Sl. Oct.

2. (US black/teen) to act in a silly or irrational manner; to overreact; to procrastinate.

[US]Da Bomb 🌐 18: Lunchin’ or Lunching: Messing up; not thinking straight; in nervous disorder; acting immaturely.
[US](con. 1986) G. Pelecanos Sweet Forever 168: Damn, I must be lunchin’.
[US]Ebonics Primer at 🌐 lunch Definition: to go crazy or to talk much shit Example: Shaquanda came over here and she lunched the fuck out!

3. (US black) to go insane.

[US]College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Lunch (verb) To go crazy.
[US]‘Master Pimp’ Pimp’s Rap 126: His wife had a massive heart attack while he was locked up. He lunched out (went insane).

In derivatives

lunching (adj.)

crazy, stupid.

[US]Wash. Post 29 Sept. F1/1–2: Of course, ‘lunching’ is the opposite of O.T.L. (out to lunch) which plainly tells you that someone is out-of-it, not-in-the-groove, or not-with-it.
[UK]M. Belmonte Compter Science and Why (1993) 🌐 A common word overheard in the halls of the high school that I attended was ‘lunshin’ [ ...] ‘Lunshin’, as it was used, described a person who was temporarily being foolish.
Online Sl. Dict. 🌐 lunching adj 1. absent minded, inattentive. 2. stupid. Origin: probably a shortened version of the phrase ‘out to lunch’ which has a similar meaning. (‘Dude, you’re so lunching.’).