Green’s Dictionary of Slang

amateur n.

1. (also E.A., enthusiastic amateur, a promiscuous young woman; thus (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].

[US]Trimble 5000 Adult Sex Words and Phrases.
[US]B. Rodgers Queens’ Vernacular 159: messer (fr pros sl) a girl who is new to prostitution; an unskilled whore. Syn: amateur.
[Aus]N. Keesing Lily on the Dustbin 41: A ‘charity moll’ is the equivalent of a World War II ‘EA’ or ‘enthusiastic amateur’: a promiscuous woman whose sexual favours are theoretically not available for sale […] [She] barters herself, instead, for presents, dinners, theatre and show tickets.
[US]Maledicta IX 148: The compilers ought to have looked farther afield and found: [...] amateur (not a pro).

2. (UK Und.) a crook who has not been in trouble with the police.

[UK]V. Davis Phenomena in Crime 134: By ‘amateur’ is meant a crook who has not been in trouble with the police.

SE in slang uses

In compounds

amateur hour (n.)

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

[US]T. Runyon In For Life 33: We never tried to rob a bank during ‘amateur hour’ — at noon, when all bankers seemingly expected to be robbed and prepared for it.
amateur night (n.) [orig. theatrical use, when on special nights amateur hopefuls were encouraged to ‘try their stuff’ on a real stage]

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

[US]M. McCall Maisie [film script] Hey, what is this? Amateur night? There’s a doctor here [HDAS].
[US]R. Stone Hall of Mirrors 105: ‘WUSA is a wel planned business operation’ [...] ‘You mean it isn’t amateur night.’ Rheinhardt said. ‘No,’ Jack Noonan said, ‘it isn’t amateur night’.
[US]J.G. Dunne True Confessions (1979) 319: She must have done it with pliers and a screwdriver. It was real amateur night.
‘W.T. Tyler’ Shadow Cabinet 16: Now it’s amateur night in Washington – four years of it.
D. Rhoden ‘Merry Misbehavior!’ in Manslife mag. n.p.: Well, lads, now that you’re past Christmas, you’re a-comin’ up on Monsieur Misbehavior’s most least favorite holiday of the year. Amateur Night.
[US] Tucson Weekly [Internet] Amateur night is what the drunks call New Year’s Eve.