under someone's thumb
Controlled or dominated by someone, as in He's been under his mother's thumb for years. The allusion in this metaphoric idiom is unclear, that is, why a thumb rather than a fist or some other anatomic part should symbolize control. [Mid-1700s]
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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.