De sexto

De sexto is short for de sexto grado or de sexto año, which literally mean "of sixth grade". It is used to refer to an action that even though it has the intention of taking advantage of someone, is naive. Its origin is probably the fact that sixth grade kids try to take advantage of younger kids in elementary school, but from an adult perspective it lacks experience and sophistication. 

Example:

Alejandro: Me la quiso hacer de sexto. Quería mandarme por cigarros para quedarse solo con la reinita. Al chile una más de estas y le surto la receta. Pobre pendejo.
Saúl: Aliviánate master. No hagas bilis.

Alejandro: He wanted to fool me in such a naive way. He wanted meto get cigarettes so that he could stay alone with that cute chick. Honestly, next time I will kick his ass. Loser.

Saúl: Easy dude. Don't get all upset.