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When someones mum wont let them do something, even though they are old enough to decide for themselves.

Overbearing, controlling mother.

Similar to being under your girlfriend/wifes thumb.(not cool)
"No way dude, Dave wants to get a tattoo and although he's 19, his mom wont let him"
"He's so under the Mum thumb!"
by nicko2 September 07, 2007
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