button one's lip

Verb

 * 1)  To remain silent, especially in order to keep a secret or to avoid saying something inappropriate.
 * 2) * 1910, Jim Nasium, "|+buttoned+|+buttons+|+buttoning+his-lip&hl=en The Baseball League on the Styx," The Sunday Press (Pittsburgh), 15 May, p. 2 (retrieved 19 July 2010):
 * "Yes," put in Solomon. "I've been tipped off to this guy Mars, and you'd better tell him to button up his lip and not start anything."