Green’s Dictionary of Slang

first of May n.2

also first-of-Mayer

(US tramp) a novice; see cit. 1931; also in ext. use as anyone who does not stay the course.

[US]Irwin Amer. Tramp and Und. Sl. 73: FIRST OF MAY.–Properly, any one newly employed by a circus, where the season starts about the first of May. By adoption, any tramp but newly arrived in a ‘push’ or new to tramp life and as yet inexperienced.
[US]J.E. Dadswell Hey, Sucker 15: If you do stop, you’re a ‘heel’ – or, worse, a ‘First of May!’.
[US]H. Gold Man Who Was Not With It (1965) 100: Hey, you first-of-Mayers, Bud Williams is back!
[US](con. 1940s) E. Thompson Tattoo (1977) 573: Know what they call you behind your back? [...] ‘First of May.’ That’s an old carnival term for someone who comes out in the spring but doesn’t last through the winter.