H.M.C. Stands for "Heavy Metal Club". It's a group/club with members who wear colors just like a motorcycle club, and have by-laws and a hierarchy. Most are local while the bigger, more recognized ones are international and have chapters throughout the world.
Dude, the local H.M.C. were all at the show last night in full regalia.
Originating from random mathletes bored during the otherwise dull math meets which they attended, the phrase "HMC" is an abbreviation for "Hot Math Chick." Mathletes have been known to use the acronym sarcastically or more rarely seriously.
Man, check out the size of the polynomials on that HMC!