Green’s Dictionary of Slang

lemon adj.1

[lemon n.]

1. of a black person, light-skinned.

[US]Irving Jones ‘Honey, You’ve Got to Leave’ [lyrics] There’s a lemon colored lady, the coons call flat-foot Sadie.
[US]Thomas S. Allen ‘Let Me Be Your Lemon Coon’ [lyrics] Ebenezer Johnson was a yaller coon.
[US]C. McKay Home to Harlem 58: A lemon-colored or paper-brown pool-room youngster.
[US]P. Oliver Blues Fell this Morning 80: To differentiate between their many shades of colour [...] ‘brightskin,’ ‘high yaller’, ‘lemon’.

2. (US) useless, second-rate.

[US]T.A. Dorgan in Zwilling TAD Lex. (1993) 54: Lemon idea.
[US]G. Henderson Keys to Crookdom 410: Lemon. Something worthless.