Symbols born of a time and place are a sort of wink-wink, you get me? shorthand. Sk8rgrl...

I suppose hobo symbols are, in some sense, as old as the concept of language itself. We may view prehistoric cave-drawings as some kind of self-expression; but they may have enormously practical implications for the day.

As far as universal symbols goes: the skull and crossbones is pretty universal for defining minefields, poisonous water or material, etc.

There's an effort to define symbols that will deter humans from digging into nuclear waste dumps ... tens of thousands of years from now.

And then there's the graffiti of street gangs, which has more immediate implications.