Hi Dear , As per your query you have pimple under armpit , Well there can be many reasons for pimples in armpit , But it seems to me that in your case the lump is cyst . An underarm cyst is typically a sac beneath the skin containing pus, like a pimple. They often result from a hair follicle or an oil-producing gland within the follicle becoming damaged. I would suggest you to not to pop them with hands as the will get further worse , Apply warm wet compresses 2-3 times a day and if it doesn t get improve then consult doctor for proper treatment and rule out condition like pimple or lymph node . Doctor may inject steroid in it or prescribe antibioticsHope your concern has been resolved. Get Well Soon. Best Wishes, Dr. Harry Maheshwari along with anti inflammatory .