"Unusually curved fingertips and nails
Clubbing of the fingertips means the tissue beneath the nails thickens and the fingertips become rounded and bulbous. The fingernails curve over the rounded fingertips.
Clubbing is thought to result from increased blood flow to the fingertips. It can run in families and be entirely harmless.
However, if it suddenly develops, it may be a sign of one of many possible medical conditions including:
long-term lung disease or heart disease, such as bronchiectasis, chronic obstructive pulmonary disease or endocarditis
inflammatory bowel disease, a long-term disease that causes inflammation of the lining of the gut
stomach cancer or bowel cancer
cirrhosis (scarring of the liver)
polycythaemia, a condition where the blood is too thick
The easiest way to spot clubbed fingertips is to hold your fingers horizontally in front of you so you are looking at the nails side on. Normal nails should dip downwards towards the cuticle. With clubbed fingertips, this natural angle is gone and the nails are in line with the top of the fingers."
- Nail abnormalities - NHS website