Here's a breakdown:
* Cual: This is the informal form of "cuál," which means "which" or "what" in English.
* el: This is the masculine definite article, meaning "the."
* apodo: This means "nickname."
* de: This is the preposition "of."
* tu: This is the possessive pronoun "your."
* hijo: This means "son."
* menor: This means "younger" or "smallest."
So, the entire phrase is asking about the nickname of the speaker's youngest son.