1) To restrain or regulate. 2) A device used to restrain flow in a particular direction. 3) An opaque shielding device designed to reduce the effects of stray light on an optical system.
1548, "to disgrace," perhaps a Scottish respelling of bauchle "to disgrace publicly" (especially a perjured knight), prob. related to Fr. bafouer "to abuse, hoodwink," possibly from baf, a natural sound of disgust, like bah. Meaning "to bewilder, confuse" is from 1649; that of "to defeat someone's efforts" is from 1675. The noun sense of "shielding device" is first recorded 1881.