The word absurd was borrowed in the 1550s from Middle French absurde, which had the same definition as we know today. As we go back to Latin absurdus, however, the semantics of it shift to mean "out of tune". What's the connection? Two plausible theories exist. It could be a figurative connection, implying that something is "out of harmony" with reality or reason, or it could have something to do with hearing loss, and the bridge definition is more along the lines of "unheard of". Both make sensel the latter is reinforced a bit more by the root in absurdus, surdus, which meant "deaf" or "mute". The prefix ab- just modifies it slightly; it meant "away" or "out" and comes from the Proto-Indo-European reconstruction hepo, which could be "off" or "away". Surdus may be reconstructed back to another PIE root, swer, which in this case meant "ringing", because of how ringing could be associated with deafness.
Hello! I'm Adam Aleksic, a sophomore studying linguistics and government at Harvard University, where I founded the Harvard Undergraduate Linguistics Society. I also have disturbing interests in politics, vexillology, geography, board games, conlanging, and law.