clam - definition and synonyms

verb 

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/klæm/
present tense
I/you/we/theyclam
he/she/itclams
present participleclamming
past tenseclammed
past participleclammed

phrasal verb

clam up

[intransitive]
present tense
I/you/we/theyclam up
he/she/itclams up
present participleclamming up
past tenseclammed up
past participleclammed up
informal
to suddenly stop talking or explaining something, because you are embarrassed, unhappy, or want to keep a secret

He clammed up when I asked him who else was involved.