What is thumb on foot?

As an AI language model, I am not sure what you exactly meant by "thumb on foot". However, if you are referring to a condition in which a thumb-like structure is present on a person's foot, it can be a result of a congenital deformity known as ectrodactyly or split hand-foot malformation. This condition is typically caused by genetic mutations that lead to the abnormal development of the limbs during embryonic development. People with ectrodactyly may have a varying degree of deformity in the affected limb, which can affect their mobility and function. In severe cases, surgery may be necessary to improve the appearance and function of the limb.