There is a hard lump in the armpit, and the common causes are as follows: 1. The most common cause is lipoma, which is softer in texture, smoother in surface, clearer in boundary with the surrounding tissues, and generally better in mobility, and can be observed without affecting the daily life; 2. Lymph nodes in the armpit, which appear to be enlarged due to infections in the surrounding tissues, which are generally accompanied with pressure and pain, and are better in mobility and smoother in surface, but there is no obvious redness and swelling around them.