“Bar” means son, “bat” means daughter and “mitzvah” means commandment or law. So a bar or bat mitzvah is literally a son or daughter of commandment (i.e., one who is expected to uphold the commandments of the Torah).
I vividly remember objecting when our twin daughters were about 8 or 9 years old and asked about a Harry Potter themed b’not mitzvah (b’not is the plural of bat). I argued, “Absolutely not. Harry Potter has nothing to do with Judaism.