Green’s Dictionary of Slang

co-pilot n.

[it helps you ‘fly’; also poss. the fact that truck drivers use it to stay awake]

1. [1960s+] (drugs) an amphetamine; usu. in pl.

2. [1980s+] (drugs) two or more people taking LSD together; more usu. the second person is not taking LSD and is there to look after people.