thumb vs thumbnail

thumb and thumbnail both are nouns.

Word	Noun	Adjective	Verb	Adverb
thumb	Yes	No	Yes	No
thumbnail	Yes	No	No	No

thumb (noun)	thumbnail (noun)
the part of a glove that provides a covering for the thumb	the nail of the thumb
a convex molding having a cross section in the form of a quarter of a circle or of an ellipse
the thick short innermost digit of the forelimb

thumb (verb)	thumbnail (verb)
feel or handle with the fingers
travel by getting free rides from motorists
look through a book or other written material