"to make sorrowful," 1620s, from sad (adj.) + -en (1); earlier "to make solid or firm" (c. 1600). The earlier verb was simply sad, from Middle English saden "become weary or indifferent," also "make (something) hard or stiff," from Old English sadian, which also could be the source of the modern verb. The intransitive meaning "to become sorrowful" is from 1718. Related: Saddened; saddening.