Green’s Dictionary of Slang

out to lunch adj.

[i.e. ‘not all there’]
(orig. US campus)

1. [1950s+] crazy, eccentric, weird.

2. [1950s+] in a daze, stupid, naïve.

3. [1960s–70s] seen as alien to the peer group.

4. [1970s] absent, unavailable.

5. [2000s] intoxicated by drink or drugs; feeling very happy.