Hard painless lump in arm pit had for 2 1/2 weeks is this serious?

It might be. The armpit is the location of normal lymph nodes, and they become more noticeable when they are enlarged associated with infection or tumor. If you have had an infection nearby, this could explain it. However, the safest thing to do is see your doctor right away for a thorough examination. Then whether or not a biopsy is needed will be determined. Best wishes:)
Could be. A number of things from a swollen (infected) sebaceous gland to en enlarged lymph gland from infection/tumor Get it evaluated ASAP Hope this helps Dr Z.