The story
Coined in 1917 by the Danish linguist Jens Otto Harry Jespersen: pro- (“before”) (from the Ancient Greek preposition πρό (pró)) + σιώπησις (siṓpēsis, “taciturnity”) (from σιωπάω (siōpáō, “to be silent”)) + -σις (-sis), (-sis, suffix forming nouns of action).
Etymology from Wiktionary, CC BY-SA 4.0.