How do you get rid of a large armpit pimple?

I have the same problem you have. It's usually caused because of blocked sweat glands. Try and use a different deodorant, a natural or crystal is what I've heard that might work. Maybe its a new shirt you've worn that hasn't been washed that caused it. I've just been washing mine, cleaning it, not using the same deodorant and stuff. It should go away after awhile, if not, you should go see a doctor. You could also use vitamin E, that heals faster, I've never tried it with pimples but try and look it up.