amateur, n. — 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].

1966		Trimble 5000 Adult Sex Words and Phrases.
1972		B. Rodgers Queens’ Vernacular 159: messer (fr pros sl) a girl who is new to prostitution; an unskilled whore. Syn: amateur.
1982		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.
1986–7		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.

1941

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.

1953

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.

1939		M. McCall Maisie [film script] Hey, what is this? Amateur night? There’s a doctor here [HDAS].
1957		W.R. Burnett Underdog 68: [of inexperienced prostitutes] ‘[I]f you ever get tired of amateur night, give me a ring,’ said Claire, and Charley snickered.
1964		R. Stone Hall of Mirrors 105: ‘WUSA is a well planned business operation’ [...] ‘You mean it isn’t amateur night.’ Rheinhardt said. ‘No,’ Jack Noonan said, ‘it isn’t amateur night’.
1977		J.G. Dunne True Confessions (1979) 319: She must have done it with pliers and a screwdriver. It was real amateur night.
1984		‘W.T. Tyler’ Shadow Cabinet 16: Now it’s amateur night in Washington – four years of it.
1998		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.
2000		Tucson Weekly 🌐 Amateur night is what the drunks call New Year’s Eve.