Английский Этимологический словарь - guess (v.)

- c.1303, gessen "to estimate, appraise," originally "take aim," probably from Scand. (cf. Middle Danish gitse, getze "to guess," O.N. geta "guess, get"), possibly infl. by M.Du. gessen, M.L.G. gissen "to guess," all from P.Gmc. *getiskanan "to get." Sense evolution is from "to get," to "to take aim at," to "to estimate." U.S. sense of "calculate, recon" is true to the oldest Eng. meaning. Spelling with gu- is late 16c., sometimes attributed to Caxton and his early experience as a printer in Bruges. Guesswork is from 1725. Guesstimate is 1934, coined by statisticians, blending guess and estimate.