A hard lump pinched under the skin in the armpit may be caused by boils, epidermal cysts, lipomas, enlarged lymph nodes in the armpit and other diseases. 1. Boils: Boils are caused by Staphylococcus aureus infection, and may also be induced by prolonged exposure to high temperatures, resulting in high sweat secretion. Patients may develop small red painful nodules on the skin, which may develop into hemispherical firm nodules, manifesting as a hard lump pinched under the skin in the armpit. 2. Epidermoid cysts: Epidermoid cysts are keratinized cysts within the dermis, which may be the result of skin lesions. It can appear in any part of the body as a subcutaneous nodule of medium hardness, and the patient may have a hard lump pinched under the skin in the armpit. 3. Lipoma: its etiology is not yet clear, it may be caused by heredity, systemic lipid metabolism abnormality, chronic inflammation and other diseases. Patients may have one or more lumps under the skin, which may be manifested as a hard lump pinched under the skin of armpit when it occurs in the armpit area. 4. Enlarged axillary lymph nodes: Enlarged axillary lymph nodes may be caused by infection, malignant tumor lymph node metastasis and other factors. Patients will appear in the local armpit lymph node enlargement, manifested as a hard lump pinched under the skin of the armpit. There are many reasons that cause a hard lump under the skin of the armpit, and it is recommended that patients go to regular hospitals to find out the cause of the disease and actively take relevant treatment measures.