mid-14c., of God, a king., etc., "make gracious allowance" for human frailty, etc.; late 14c., "yield deferentially," from Old French condescendere (14c.) "to agree, consent, give in, yield, come down from one's rights or claims," and directly from Late Latin condescendere "to let oneself down, stoop," in Medieval Latin "be complaisant or compliant," from assimilated form of Latin com "with, together" (see con-) + descendere "to descend," literally "climb down" (see descend).
Sense of ""voluntarily waive ceremony or dignity proper to one's superior position or rank and willingly assume equality with inferiors" is from early 15c. Generally a positive word in Middle English; the modern, negative sense is from the notion of a mere show or assumed air of condescending (compare sense evolution in patronize). Also in Middle English "give one's consent; come to mutual agreement; make a concession."