"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").*
https://www.etymonline.com/word/pregnant
(TIL, for those who'd suspect otherwise. I'm quite a fan of etymology, and Etymology Online in particular.)
https://en.wikipedia.org/wiki/Comic_timing#Pregnant_pause
(Although I've also heard it used for pauses outside of comedy)
The word pregnant is used symbolically - a woman carries a child with great effort (the symbology gets even more significant if we consider the act of sex before the pregnancy) - and the child Is born with great pain, but eventually happiness of the parents.
So this symbol of promise-of-joy-at-end-of-suffering, when combined with the symbol of the word (as carrier of meaning) gives birth (see how im using birth here to explain something? Its a similar mechanism) to a new symbol, “pregnant words” - these words written during great emotional storms, and attempt to draw a picture of immense meaning and beauty to those who read it, if they are also in the right state of mind to understand.
Even trying to truly explain what it is, is kind of difficult.
We humans actually are “complex symbol creators” - rather than “language users” - its a wonder we understand each other at all sometimes, and its no wonder that we disagree so much.
Anyway, I wasn't trying to be condescending by pointing you at a dictionary -- I'm not a native speaker myself and thought you might find it useful the next time you come across something that doesn't immediately make sense.