Wordorigins.org

Slang and Its Analogues; Farmer, J.S. and W.E. Henley; 1890-1904; Reprinted 1970 by Arno Press.

A compendium of non-standard English from the turn of the 20th century. Originally in seven volumes, it has been reprinted into one. It is comprehensive and includes usage cites. Perhaps most amazing is that it includes profanity that would not make it into standard dictionaries until the 1960s. Farmer and Henley include usage citations which have already been heavily mined by more modern dictionaries.