c. 1300, descenden, "move or pass from a higher to a lower place," from Old French descendre (10c.) "descend, dismount; fall into; originate in" and directly from Latin descendere "come down, descend, sink," from de "down" (see de-) + scandere "to climb," from PIE root *skand- "jump" (see scan (v.)).
Sense of "originate, proceed from a source or original" is late 14c. in English, as is that of "have a downward slope." Meaning "come down in a hostile manner, invade" is from early 15c. Related: Descended; descending.