WordNet sense 1 (put an end to a state or an activity):

cease, discontinue, stop, give up, lay off

WordNet sense 2 (give up or retire from a position):

resign, resign, leave office, step down

WordNet sense 3 (turn away from; give up):

relinquish, renounce, foreswear

WordNet sense 4 (go away or leave):

depart, take leave

WordNet sense 5 (give up in the face of defeat of lacking hope; admit defeat):

drop out, throw in, chuck up the sponge, drop by the wayside, fall by the wayside, throw in the towel