A hard lump under the skin of the armpit may be a swollen lymph node, sebaceous cyst, etc., and you should consult a doctor in time to find out the cause. 1. Lymph node enlargement: Lymph node enlargement is a common clinical symptom, and a variety of diseases can lead to lymph node enlargement, such as infections, tumors and other factors. Lymph node enlargement is a common clinical symptom, which can be caused by various diseases, such as infection, tumor and other factors. It can be manifested as hard and palpable lumps under the skin. 2. Sebaceous cysts: Due to the blockage of sebaceous ducts in the skin, sebum gradually accumulates and forms retention cysts. Often occurring in the head, face, back and buttocks and other parts of the cysts can exist for many years without conscious symptoms, but also susceptible to infection, pus ulceration, and easy to recur. If you find a hard lump under the skin of the armpit, it is recommended that the patient consult a doctor in a timely manner to clarify the cause of the disease and give targeted treatment.