You see this a lot when individuals who are accused of something decide to clam up, often under the advice of their attorney.
Back in California, Zach finds that people from whom he might learn about the bombing either die, disappear, or clam up.
A little general chitchat once in a while, then she'd clam up to do a little mental orbit figuring.
bivalve mollusk, c.1500, in clam-shell, originally Scottish, apparently a particular use from Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (cf. Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"). If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.
"to dig for clams," 1630s, American English, from clam (n.). Clam up "be quiet" is 1916, American English, but clam was used in this sense as an interjection mid-14c.
To stay or become silent; stand mute; button up: When I ask for details he just clams up (1916+)
clam up •The term must be earlier than the date given, although no examples can be provided. Middle English clum, ''be quiet! shut up,'' of obscure origin, may not be related to clam (1916+)