late 14c., "jaw, jawbone," from Late Latin mandibula "jaw," from Latin mandere "to chew," which is perhaps from PIE root *mendh- "to chew" (source also of Greek mastax "the mouth, that with which one chews; morsel, that which is chewed," masasthai "to chew," mastikhan "to gnash the teeth"). But de Vaan suggests a semantic development from a PIE root meaning "to stir, whirl," source also of Sanskrit manthanti "to whirl round, rub," Lithuanian mesti "to mix," Old Church Slavonic mesti, Russian mjasti "to trouble, disturb." Of insect mouth parts from 1826.
