word-forming element meaning "toward; against; before; near; across; down," also used as an intensive, from Latin ob (prep.) "in the direction of, in front of, before; toward, to, at, upon, about; in the way of; with regard to, because of," from PIE root *epi, also *opi "near, against" (see epi-).

Proto-Indo-European root meaning "to strike," an extended sense from root *per- (1) "forward, through."

It forms all or part of: compress; depress; espresso; express; impress (v.1) "have a strong effect on the mind or heart;" imprimatur; imprint; oppress; oppression; pregnant (adj.2) "convincing, weighty, pithy;" press (v.1) "push against;" pressure; print; repress; reprimand; suppress.

"weighted or pressed down," physically or mentally, late 14c., past-participle adjective from oppress.

Ovirredyn with a carte-wheel, The chyld oppressyd lay in the streete deed. [John Lydgate "Lives of Ss. Edmund & Fremund and the Extra Miracles of St. Edmund," mid-15c.] 

1640s, "unreasonably or unjustly burdensome," from Medieval Latin oppressivus, from oppress-, past participle stem of opprimere "press against, press together, press down;" figuratively "crush, put down, subdue, prosecute relentlessly" (see oppress). Sense of "inclined to oppress, tyrannical" is from 1712; that of "heavy, overwhelming" (of grief, woe, heat, etc.) is by 1712. Related: Oppressively; oppressiveness.

"one who exercises undue severity in the use of power or authority," c. 1400, oppressour, from Old French opresseor, from Latin oppressor "a crusher, a destroyer," from opprimere (see oppress (v.)). In Middle English also "a criminal; a rapist" (mid-15c.).