like a bump on a log - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]