thumb (Noun) — The thick short innermost digit of the forelimb.
thumb (Noun) — The part of a glove that provides a covering for the thumb.
thumb (Noun) — A convex moulding having a cross section in the form of a quarter of a circle or of an ellipse.
thumb (Verb) — Travel by getting free rides from motorists. ex. "You can, of course, thumb anywhere that there are cars and traffic"
thumb (Verb) — Look through a book or other written material. ex. "He thumbed through the report"
thumb (Verb) — Feel or handle with the fingers. ex. "thumb the binding of the book"