I'm thinking of using my roomates coffee mug when he was gone one weekend and instead of washing it and putting it back I stuffed it in my underwear drawer and havent touched it for months
That's disgusting! Why would you put the mug in your underwear drawer?
I was too lazy to wash it.
It's fine that you used his mug. But when you were done, you should have cleaned and returned it where you found it. You might have a bad relationship with your roommate if you continue to do this sort of thing.
He should stock up.
He doesn't have to get more if you'd just take care of it when you are done. It's really easy to wash a mug and just takes a minute.