a derogatory term for Ford Motor Company's legendary sporty car, the Mustang. The implication is that the Mustang is a dog (mutt) that stinks (stank.) Usually used by fans of General Motors' late lamented Glimmer Twins, the Camaro and the Firebird.
gene: so whatcha gonna do now that yer ex-wife gotcher Explorer in the divorce?
dean: i dunno. i always wanted a Mustang, maybe I'll get a red one.
gene: whatthefuck? why bother with a Muttstank? you might as well walk, or buy a Hyundai.
dean: stfu, at least they didn't discontinue the Mustang like they did the Cramped Arrow or the Misfirebird.
me: wait till they bring back the Challenger!!
gene and dean: who said that?