Old English dreorig "sad, sorrowful," originally "cruel, bloody, blood-stained," from dreor "gore, blood," from (ge)dreosan (past participle droren) "fall, decline, fail," used of rain, snow, dew, fruit, and the slain, from Proto-Germanic *dreuzas (source also of Old Norse dreyrigr "gory, bloody," and more remotely, Old Saxon drorag, Middle High German troric "bloody;" German traurig "sad, sorrowful"), from PIE root *dhreu- "to fall, flow, drip, droop" (see drip (v.)).
The word has lost its original sense and the notion of "dripping blood." Sense of "lonesomely dismal, gloomy" first recorded 1667 in "Paradise Lost," but Old English had a related verb drysmian "become gloomy." Weakened sense of "causing a feeling of tedium, tiresomely monotonous" is by 1871. Related: Drearily.