thumb vs riff

thumb and riff both are nouns.

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

As verbs, thumb and riff are synonyms defined as:

Other synonyms of thumb include flick, flip, leaf, riffle.

thumb (noun)	riff (noun)
the part of a glove that provides a covering for the thumb	a jazz ostinato; usually provides a background for a solo improvisation
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)	riff (verb)
feel or handle with the fingers	play riffs
travel by getting free rides from motorists	look through a book or other written material
look through a book or other written material