Acronym for How Much A Month. Typical lower class people with questionable credit ratings don't care about loan term or interest rates, all they care about is How Much A Month it costs.
Can also be used as an verb in other situations.
My white trash neighbors just HMAM'd a new camper and boat.
I HMAM'd some new tires on my Sears card.
I'd HMAM the shit out of that blond chick for sure.
Prices shown in USD.