A caitiff is a contemptible or cowardly person. It’s archaic, so if you say it with a straight face you might even get away with the insult. It’s also a word that — like many in the lexicon — has come down in the world. It started out, sometime before 1300, to mean a captive. It came through French from Latin captivum, with the same sense (captive comes from the same Latin word by a later re-borrowing, again through French, so it and caitiff make a doublet).
As captives were not in the best of circumstances, caitiff began to mean a wretched or miserable person. Chaucer uses it several times in the Canterbury Tales, as in the Knight’s Tale: “And now I am so caitiff and so thrall / That he that is my mortal enemy / I serve him as his squier poorely”. [thrall: enslaved; squier: squire].
The sense then shifted further towards contempt, implying a mixture of misery and wickedness, and then to the sense of cowardly. Later it became a staple of those historical writers seeking to gain some antique credibility through choice of language. Sir Walter Scott comes especially to mind:
The caitiff villain yet seemed, amid his hardened brutality, to have some sense of his being the object of public detestation, which made him impatient of being in public, as birds of evil omen are anxious to escape from daylight, and from pure air.
The Heart of Mid-Lothian, by Sir Walter Scott, 1818.