The habit of putting a thumb in one's mouth and sucking it in rhythm. Inherently done by babies as a reflex, the same one responsible for [[breastfeeding]].

It is weaned usually at age 5, when children start to grow permanent teeth. Anyone older than 5 sucking thumbs usually has a psychotic or mental disorder or just does it as a habit, especially either to relieve a hurting/swollen thumb or consciously wanting to imitate the experience of a second childhood.