Five. Five dollar. Five dollar not-even-a-good-deal!!!

The B.M.T. in Subway’s Italian B.M.T. stands for Biggest, Meatiest, Tastiest (LAME name). In the early days of SUBWAY® restaurants, it was a promotion that was based on the Brooklyn Manhattan Transit system in New York City that went along with the subway theme.

Source: Lemm told me this around Halloween time, but had his facts wrong. I found the answer here, and followed the link to here.