err / ər; er/ • v. [intr.] formal be mistaken or incorrect; make a mistake: the judge had erred in ruling that the evidence was inadmissible. ∎ [often as adj.] (erring) sin; do wrong: the erring brother who had wrecked his life.PHRASES: err on the side of display more rather than less of (a specified quality) in one's actions: it is better to err on the side of caution.