Dear Boya.
If it is a superficial swelling (arising from skin or just deep to skin), then most probably it a fat lump (lipoma) or a cyst (sebaceous/ dermoid). If it is there for years not showing any sign or rapid progession in size (slow increase in size is usually not problamatic), then it must be a benign swelling (non cancerous).
If it is deeper in location or in the armpit (? lymph node) then consult a doctor or get the FNAC done to confirm the diagnosis.
Local examination will be very helpful even a fotograph of the swelling can be of some help.
Best wishes
Dr Tewari (India)
sounds like a benign fatty lump called a lipoma
check it with your GP/PCP - he might want a surgeon to see it
a lipoma often needs no treatment