The word guacamole was borrowed from Spanish in the early twentieth century, and they took that from the Aztecs and their word ahuacamolli. That's a portmanteau of ahuacatl, meaning "avocado", and molli, meaning "sauce" (this is also the etymon of the Mexican sauce mole). Makes a lot of sense. I've already covered ahuactl before, but it had a curious secondary definition of "testicle", due to visual resemblance. The eventual root is thought to be Proto-Aztecan pawa, also referring to the fruit. Molli, meanwhile, probably took a similar route but we don't have a reconstruction for it. The shortening guac (which is largely limited to North American colloquial speech) was spawned in the early 1980s, and the expression holy guacamole modified the pre-existing holy moley, which has been around since the 1940s and referred to a magical herb from Greek mythology.
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.