De piquete de ombligo

De piquete de ombligo is a way to describe a relationship. It literally means 'of poking each others belly button'. Two or more people are said to have a de piquete de ombligo relationship if they get along well and are really close. Mexicans don't go by the streets poking random strangers' belly buttons. They only do it to a rather selective and small circle of friends (and foes). The expression is typically preceded by llevarse (lit. to carry or behave themselves) as in llevarse de piquete de ombligo.

Example:

Profesor: No me gusta darme mi taco en el salón de clases. Prefiero una relación abierta y de amistad con mis estudiantes, de piquete de ombligo. 

Coordinador: Nada más no vayas a picarle el ombligo a ninguna alumna. Si no, te cae la voladora.

Instructor: I do not like to pretend to be unreachable in the classroom. I rather developing a friendship with my students, really casual and familiar.

Coordinator: Just make sure you don't actually poke their belly buttons. Otherwise you'll face disciplinary measures.