WordNet (r) 3.0 (2006):

hang in
    v 1: be persistent, refuse to stop; "he persisted to call me
         every night"; "The child persisted and kept asking
         questions" [syn: persevere, persist, hang in, hang
         on, hold on]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "hang in":
   abide, abide with, bear, bear up, bear with, bide, brave, brook,
   come up fighting, dig in, endure, get along, get on, go on,
   go on with, hang in there, hang on, hang tough, hold fast, hold on,
   hold out, hold up, keep at it, keep up, live through it,
   live with it, lump, lump it, make out, manage, never say die,
   not flag, not give up, not weaken, persevere, persist, put up with,
   remain firm, see it out, see it through, stand, stand fast,
   stand firm, stand for, stand no nonsense, stand the gaff, stand up,
   stay it out, stay the distance, stay with it, stick, stick fast,
   stick it, stick it out, stick out, stick to it, stick with it,
   suffer, support, survive, sustain, sweat it out, take it,
   take up with, take what comes, tolerate, tough it out