When the word lobster was introduced into English about a thousand years ago, it was spelled loppestre, and through Chaucer's time it was consistently spelled with a p in the middle. The b we use today probably replaced it in the sixteenth century due to influence from the Old English word lobbe, meaning "spider". Loppestre is thought to be a corruption of Latin locusta, which meant both "lobster" and "locust" and is also the etymon of the word locust (the connection between the two was a perceived physical resemblance). Nobody really knows where locusta comes from, but it's thought to be related to lacerta, meaning "lizard". Usage frequencies of the words lobster and locust were pretty evenly increasing until the 1960s, when lobster got a lot more popular and locust began decreasing.
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.