There are several types of cardinals, and all of them have the same origin. The sense of cardinal as a type of bird originated in the 1600s when they were discovered, and was named so because the birds' plumages resembled the robes of a religious cardinal. The word cardinal as pertaining to the ecclesiastical rank derives from the Latin word cardinalis, meaning "principal and essential". It is obvious why this would come to be; religion and religious authorities were paramount at the time. Coincidentally, it is from this same word (cardinalis) that the word cardinal pertaining to direction originated from (think cardinal points on a compass rose), since those directions were about equally significant to daily life at the time. Now, since cardinalis had a meaning of "pivotal", this next change kind of makes half-sense: the term is a portmanteau of the Latin word for "hinge" (cardo, since a hinge is pivotal too) and the irrelevant suffix -alis. This is interesting as it is, but countless possibilities open up with the fact that where cardo comes from is unknown. One interesting theory would make cardo a cognate with the English word hinge: it is possibly from the Proto-Indo-European word nerd ("swing"). Other conjectures follow this to Greek, but we'll never know for sure by which way fate swung cardinal our way.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.