be condemned out of one's own mouth; be condemned on one's own showing; be unable to advance any further arguments to justify oneself; fall silent on finding oneself bested in argument; find oneself devoid of all argument; have nothing left with which to justify oneself; have nothing to say [be tongue-tied] on realizing one's own shaky stand; not have a leg to stand on; shut up when defeated in argument; unable to find a word to justify oneself:
As he had a weak case and could not defend himself, he had to resort to sophistry.