rat (informal)
n
1 bad lot, bastard (offensive) bounder (old-fashioned Brit. slang) cad (old-fashioned Brit. informal) heel (slang) ratfink (slang, chiefly U.S. and Canad.) rogue, rotter (slang, chiefly Brit.) scoundrel, shit (taboo slang) shyster (informal, chiefly U.S.)
2 betrayer, deceiver, defector, deserter, double-crosser, grass (Brit. informal) informer, nark (slang) quisling, snake in the grass, stool pigeon, traitor, two-timer (informal)
vb
3 (with)
on abandon, betray, defect, desert, do the dirty on (Brit. informal) leave high and dry, leave (someone) in the lurch, run out on (informal) sell down the river (informal)