Green’s Dictionary of Slang

amateur n.

1. [1910s+] (also E.A., enthusiastic amateur, a promiscuous young woman; thus [1920s+] (orig. US) lose one’s amateur standing/status v., to move into the world of professional, full-time prostitution [the term depends on the assumption that for women any sex before marriage was tantamount to prostitution].

2. [1930s–40s] (UK Und.) a crook who has not been in trouble with the police.

SE in slang uses

In compounds

amateur hour (n.)

[1950s] (US Und.) noon, as a time when banks appear to expect to be robbed.

amateur night (n.) [orig. theatrical use, when on special nights amateur hopefuls were encouraged to ‘try their stuff’ on a real stage]

[1930s+] (orig. US) an exhibition of more than usual ineptitude, esp. by one who is supposedly more competent.