1540s, "words, clauses, etc. inserted into a sentence," from Middle French parenthèse (15c.), from Late Latin parenthesis "addition of a letter to a syllable in a word," from Greek parenthesis, literally "a putting in beside," from parentithenai "put in beside," from para- "beside" (see para-(1)) + en- "in" + tithenai "put, place" (see theme). Sense extension by 1715 from the inserted words to the curved brackets that indicate the words inserted.