[1] : WordNet (r) 2.0

clam up
     v : refuse to talk or stop talking; fall silent; "the children
         shut up when their father approached" syn: close up, dummy
         up, shut up, belt up, button up, be quiet, keep
         mum ant: open up
see also:
close up dummy up shut up belt up button up be quiet 
keep mum open up 

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