late 14c., "act of going back," from Latin regressus "a return, retreat, a going back," noun use of past participle of regredi "to go back," from re- "back" (see re-) + gradi "to step, walk" (from PIE root *ghredh- "to walk, go").
1550s, "to return to a former state," from Latin regressus (see regress (n.)). Meaning "to move backward" is from 1823. The psychological sense of "to return to an earlier stage of life" is attested from 1926. Related: Regressed; regressing.