Green’s Dictionary of Slang

1. (US) a general term of abuse.

1899–1900

C.L. Cullen Tales of the Ex-Tanks 265: Council bluffs is a clammy looker, and all that.

2. (US) foolish, stupid.

1934

(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 339: ‘That guy’s clammy,’ Studs said.