No separate word; use finger.
under the thumb of: met., use adj., P. and V. ὑπήκοος (gen. or dat.), ὑποχείριος (dat.); or prep., P. and V. ὑπό (dat.).
thumb (a book): use Ar. and P. πατεῖν (Ar. 471; Plato, Phaedrus 273A).