There are two that I can think of that have always made me wonder their origin (these may be American English phrases, I'm not sure):
Sounds like Greek to me.
Sometimes said when someone doesn't understand something. If someone spurts out Calculus equations, one who doesn't understand might say such a phrase.
Excuse my French.
Typically said when a speaker curses or says something inappropriate. This is a lighthearted way of apologizing. This one has always boggled me— why French?
Does anyone know the origins of these phrases? Do you ever say them yourself? They seem a bit old-fashioned, but on occasion I've caught myself using them.
Are there any other phrases that do this? In English or any other language?