Green’s Dictionary of Slang

-a sfx

[20C+] used to denote a colloq. or slangy pron. of:.

(a) have, e.g. coulda, musta, shoulda, woulda.

(b) to, e.g. gonna, gotta, oughta, wanna.

(c) of, e.g. cuppa n., lotsa n., lotta n.