tumbler n.

mid-14c., "acrobat," agent noun from tumble (v.). Compare Old English tumbere "tumbler, dancer." A fem. form was tumblester (early 15c.), tumbester (late 14c.) "female acrobatic dancer." Meaning "drinking glass" is recorded from 1660s, originally a glass with a rounded or pointed bottom which would cause it to "tumble;" thus it could not be set down until it was empty. As a part of a lock mechanism, from 1670s.