>Funny, I thought you were talking French, but my freind explained to me that is was in Spanish (though he is not sure about El gran bonete)
>
>http://www.folkloredelnorte.com.ar/jbonete.htm

It is Spanish, there is a link further down your googled search that explains a little bit the phrase http://forum.wordreference.com/showthread.php?t=732563

The original poster's question was (quick and dirty translation)

Does anyone know an English expression similar to "El gran bonete" ("big cap")
I am not sure if the expression is known in Spain and the rest of Latin America, but in Argentina is used in a comic or sarcastic way when it is not known who was responsible for a determined action.
For example, the mother arrives home and founds a broken vase on the floor, when the mother confronts the kids to learn who broke the vase, the kids deny involvement, so the mother says "... if you didn't do it who did it? "El gran bonete?"