like a bump on a log
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]
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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.