To you, the word diagnosis probably evokes images of doctors in lab coats delivering grim news. The term can, however, refer to any kind of scientific identification, and that definition was more prominent when the word was borrowed from medical Latin in the 1680s. That eventually traces to Ancient Greek diagingoskein, a verb meaning "to discern", which is still tied very closely to the modern denotation. This is where it gets interesting, though: diagingoskein can be broken down into two words: dia, "between" or "apart", and gignoskein, "to learn". Since there are two objects involved in both definitions of dia, it makes sense that it would come from Proto-Indo-European dwo, meaning "two" (also making it the relative of two, di-, and duo-). Gignoskein, meanwhile, came from PIE gno, "to know". So diagnosis can be interpreted to mean "knowing two" or "learning apart".
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd