Green’s Dictionary of Slang

(Ulster) a sly, untrustworthy person.

1977		in D. Marcus Best Irish Short Stories 29: Slippy-tit in person. Standing straddle- legged beneath the cliffs. Drying his dirty carcase.
1983		P. Boyle Port Wine Stain 197: Over all these years Mister Slippy-tit has been codding the natives with his bragging and boasting.