Parentheses have been used since right before the fifteenth century as a method to insert more information in a text, but the word for those brackets wasn't first attested until the 1540s, when it was taken from Latin parenthesis. This actually referred to the addition of only one letter to a syllable at the time of the Romans, but was repurposed for the new linguistic invention. The Latin word comes from Greek, where para meant "beside", en meant "in", and thesis was related to a verb meaning "to put". Together, a parenthesis "put something in beside" some writing. Para is from Proto-Indo-European per ("before"), en has always kind of looked like that, and thesis derives from the reconstuction deh, which could also mean "to place" (I'm just going to put this parenthetical in beside this blog post).
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.