Is it ensure, insure, or assure?
late 14c., mocchen, imitative (cf. crunch), or perhaps from Old French mangier "to eat, bite," from Latin manducare "to chew." Related: Munched; munching.
To eat, esp to consume hungrily; pig out: You'll get together with your friends, right, and you'll be munching up or something/ She knew she shouldn't be munching out on carbos like this