"convincing, weighty, pithy," late 14c., "cogent, convincing, compelling" (of evidence, an argument, etc.); sense of "full of meaning" is from c. 1400. According to OED from Old French preignant, present participle of preindre "press, squeeze, stamp, crush," from earlier priembre, from Latin premere "to press, hold fast, cover, crowd, compress" (from PIE root *per- (4) "to strike"). But in Barnhart it is from Latin praegnans "with child," literally "before birth" and thus identical with pregnant (adj.1).
Retained its status as a taboo word until c. 1950; modern euphemisms include anticipating, enceinte, expecting, in a family way, in a delicate (or interesting) condition. Old English terms included mid-bearne, literally "with child;" bearn-eaca, literally "child-adding" or "child-increasing;" and geacnod "increased." Among c. 1800 slang terms for "pregnant" was poisoned (in reference to the swelling).