The knave of clubs, short for Pamphile, the French word for the knave of clubs.
“Dr. Johnson's derivation of Pam from palm, because `Pam' triumphs over other cards, is extremely comic. Of course, Pam is short for Pamphile, the French name for the knave of clubs.” —Notes and Queries (W. W. Skeat, 1 May, 1886), p. 358.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894