WordNet (r) 3.0 (2006):

butt in
    v 1: break into a conversation; "her husband always chimes in,
         even when he is not involved in the conversation" [syn:
         chime in, cut in, put in, butt in, chisel in,
         barge in, break in]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "butt in":
   barge in, blow it, break in, break in upon, burst in, busybody,
   charge in, come between, crash, crash in, crash the gates,
   creep in, crowd in, cut in, edge in, elbow in, encroach, entrench,
   foist in, fool, go off half-cocked, horn in, impinge, impose,
   impose on, impose upon, infiltrate, infringe, insinuate, interfere,
   interlope, interpose, interrupt, intervene, intrude, invade,
   irrupt, make, monkey with, obtrude, press in, push in, put on,
   put upon, rush in, slink in, slip in, smash in, sneak in,
   speak inopportunely, speak too late, squeeze in, steal in,
   storm in, tamper with, throng in, thrust in, trench, trespass,
   work in, worm in