There is this expression in Spanish: pasarlo teta. It means to have a really good time:
Ayer fuimos al parque de atracciones y nos lo pasamos teta.
Yesterday we went to the theme park and we spent it boob
To my understanding, it is used in contexts when you enjoy like a child, but not always is this case.
So I am wondering: why is this expression used? What is its origin? Is it related to the pleasure of an infant while breastfeeding?
It is very difficult to track the origin and motivations of very recent expressions, due to the information going spread faster and faster. In this case, the expression is not present in the CORDE, but it is in the CREA. The first use appears in a 1981 book, "Días de guardar", from Spanish writer Carlos Pérez Merinero. Examples:
"Henri era un cliente asiduo de "Le Pelican" y solíamos jugar al juego del camarero y el cliente indeciso con destreza de maestros. Sólo que entonces tenía yo el papel del cuchillero y él el de la víctima propiciatoria. Nos lo pasábamos teta."
"El cabrón del maître observa la jugada y se lo está pasando teta. A lo mejor el muy hijoputa, al ver mi pasividad, me toma por un mariposón."
The Spanish language has a variety of expressions with the same structure to note that someone is having a good time. Every one of them uses a positive word (in some sense or another), related to entertainment:
Most of these expressions have been recently included in the DRAE. Some of them are from the 2001 version, others from the 1992 version. So if you think about it, using "teta" is just a normal evolution, an expression that just nobody knows its origin but fell in grace among the people, because it represents both the child being breastfeeded (related to entertainment and joy) and the adult, more sexual theme of playing with a woman's breasts. But that may place the expression a bit on the machismo side, the same case as something cojonudo being something good, and a coñazo being something bad, only on a lighter level. But this sexual theme regarding the origin of the expression does not mean that you have to use it in sexual contexts, as seen in the examples above, so I am more inclined to think that the expression may have its origins in the entertainment and joy of breastfeeding, as you propose in the question.
I never heard it before... but "teta" is "boob". You like boobs, right? But it must be an expression from a certain area. A more neutral expression will be "la pasamos genial", o "la pasamos increìblemente bien". In some other places (like here, Arg) you could say "la pasamos bomba" -as someone pointed-, and if you ask why bombs are funny... well, they surely are not, but it means that some event was "explosive" in terms of how funny it was, or how uncontrolled in terms of drugs/sex.
