When you call the alphabet "the ABCs", it's etymologically redundant. Our word alphabet is from Latin alphabetum, from Greek alphabetos, both of which were also used to denote a set of letters in a language. Now, the origin here is blaringly obvious: alphabetos is a portmanteau of alpha and beta, the first two letters of the Greek alphabet that we still use for military designations, naming pack animals, describing particles, and the like today. Basically it amounts to calling your ABCs just the ABs. The Alpha letter and word for the letter are both from the Phoenicians (the dudes who had the first phonemic alphabet), where alpha took the form of alb and most likely came from Proto-Semitic, because it may be philologically compared to the first letters of the Arabic and Hebrew alphabets (alif and alef, respectively). Beta followed a similar route: it is from the Phoenician letter beth. Alphabet was introduced in the fifteenth century and replaced an English word meaning "row of letters".
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.