ritardando (adv.)

early 15c., retardacion, "fact or action of delaying or making slower in movement or time," from Latin retardationem (nominative retardatio) "a delaying," noun of action from past participle stem of retardare "to make slow, delay, keep back, hinder," from re- "back" (see re-) + tardare "to slow," related to tardus "slow, sluggish" (see tardy).

The psychological sense of "educational slowness, educational progress slower than average for a group" is from 1907, perhaps a back-formation from retarded. For the meaning "act of retarding," retardment also was used (1640s).