*per- (4) Look up *per- (4) at Dictionary.com
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; pregnant (adj.2) "convincing, weighty, pithy;" press (v.1) "push against;" pressure; print; repress; reprimand; suppress.