1
general::
Phrase(s): like a bump on a log
Fig. completely inert. (Derogatory.) • Don’t just sit there like a bump on a log; give me a hand! • You can never tell what Julia thinks of something; she just stands there like a bump on a log.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Unmoving, inactive, stupidly silent. For example, Harry just sat there like a bump on a log while everyone else joined in the fun. This simile presumably alludes to the immobility of such a protuberance. [Colloquial; mid-1800s]
American Heritage Idioms