1275-1325;Middle Englishengros(s)en to gather in large quantities, draft (a will, etc.) in final form < Anglo-Frenchengrosser, partly < Medieval Latiningrossāre to thicken, write large and thick (Latinin-in-2 + gross(us) thick + -āre infinitive suffix); partly < Anglo-French,Middle Frenchen gros in quantity, wholesale < Latinin + grossus; see gross
C14 (in the sense: to buy up wholesale): from Old French en gros in quantity; C15 (in the sense: to write in large letters): probably from Medieval Latin ingrossāre; both from Latin grossus thick, gross
c.1300, from O.Fr. en gros "in bulk, in a large quantity, at wholesale," as opposed to en detail. Figurative sense of "absorb the whole attention" is first attested 1709. A parallel engross, meaning "to write (something) in large letters," is from Anglo-Fr. engrosser, from O.Fr. en gros "in large (letters)." Related: Engrossed; engrossing.