It's a corruption of the French word Manger meaning 'to eat'. During the first world war Australian soldiers found pronunciations of critical French words hard to master.
"Munchies mate" pointing at mouth and smacking lips to bewildered French shop keepers and café owners.
When your hungry as fuck after smoking some bud thats mad good.
"yo you see him? he put yo mommas cornbread muffin a charleston chew in the pockets of his sweats then grabbed a glass of oj...hes got some rad munchies.