O.E. sæd "sated," from P.Gmc. *sathaz (cf. O.N. saðr, M.Du. sat, Du. zad, O.H.G. sat, Ger. satt, Goth. saþs "satiated"), from PIE *seto- (cf. L. satis "enough, sufficient," O.C.S. sytu, Lith. sotus, O.Ir. saith "satiety"), from base *sa- "satisfied" (cf. Skt. a-sinvan "insatiable"). Sense development seems to have passed through a meaning "heavy," and "weary, tired of" before emerging c.1300 as "unhappy." An alternative course would be through "steadfast, firm," and "serious" to "grave." In the main modern sense, it replaced O.E. unrot, negative of rot "cheerful, glad." Slang sense of "inferior, pathetic" is from 1899; sad sack is 1920s, popularized by World War II armed forces (specifically by cartoon character invented by Sgt. George Baker, 1942, and published in U.S. Armed Forces magazine "Yank"), probably a euphemistic shortening of common military slang phrase sad sack of shit. The verb sadden "to make sorrowful" is from 1600; earlier form was sade, from O.E. sadian.