> what English dialect it's from?

I assume American English.

Prior to the sexual slang made popular by the movie Clerks, snowballing in the context I used basically means to blindside or con someone.

One definition on Urban Dictionary:

"A situation where a criminal has found themselves in possession of an easy target and proceeds to rob them and leave them mortally wounded for fun, a synonym for getting iced."

There's also snowballing meaning a problem getting bigger and bigger when unaddressed. I'm probably using it in an older, not-exactly-mainstream way.

In my dialect, which is some sort of American English, trying to snow someone means trying to BS or con them.

Blindsiding someone means taking them unawares - hitting them when they aren't looking, physically or metaphorically.

A speedball has cocaine in it, which is sometimes known as snow.

"Iced" can mean killed, but the Urban Dictionary definition is oddly specific

I agree about "snowballing" meaning increasing in size or momentum, but it doesn't have to refer to a problem.

Quite literally. I assume you never got a snowball in you eyes?