Green’s Dictionary of Slang

casual; thus casualty-boy, a boy who hires himself out to a costermonger or market greengrocer.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor I 33/2: Some lads, however, are the smallest class of costermongering youths; and are sometimes called ‘cas’alty boys,’ or ‘nippers’ [Ibid.] 64/1: He is mostly engaged in selling dried haddocks and red herrings, and other ‘cas’alty fish that may come across him’.