"fungal decay in timber," by 1779, from dry (adj.) + rot (n.). Figurative sense of "concealed or unsuspected inward degeneration" is by 1821.
Entries linking to dry rot
dry adj.
Middle English drie "without moisture, comparatively free from water or fluid," from Old English dryge, from Proto-Germanic *draugiz (source also of Middle Low German dröge, Middle Dutch druge, Dutch droog, Old High German trucchon, German trocken, Old Norse draugr), from Germanic root *dreug- "dry."
Meaning "barren" is mid-14c. Of persons, "showing no emotion," c. 1200; of humor or jests, "without show of pleasantry, caustic, sarcastic" early 15c. (implied in dryly). Sense of "uninteresting, tedious" is from 1620s. Of wines, brandy, etc., "free from sweetness or fruity flavor," 1700. Of places prohibiting alcoholic drink, 1870 (dry feast, one at which no liquor is served, is from late 15c.); colloquial dry (n.) "prohibitionist" is by 1888, American English political slang.
Dry goods (1650s) were those dispensed in dry, not liquid, measure. Dry land (that not under the sea) is from early 13c. Dry-nurse "nurse who attends and feeds a child but does not suckle it" is from 1590s. Dry run "rehearsal" is by 1941. Dry ice "solid carbon dioxide" is by 1925.
rot n.
early 14c., "decay, corruption, putrefaction," from rot (v.) or of Scandinavian origin (compare Icelandic rot, Swedish röta, Danish røde "decay, putrefaction"), or both, in any case from the root of the verb. From c. 1400 as the name of a disease in sheep, also generally, "condition of rottenness in a plant or animal, process or state of being rotten." Slang sense of "rubbish, trash" is from 1848.