"Never slap a man who's chewing tobacco"

It's a quote by Will Rogers, and almost anything he said (in one sentence or less) could be described as a proverb. The meaning is sort of obvious: if you slap a man who is chewing tobacco, you are likely to end up wearing it!

It would be similar to "catching a tiger by its tail". The idea being that you should not underestimate your adversary.

About Will Rogers

It means that if a man is doing something undesirable, don't confront him upfront or you'll be negatively affected in some way or the other too.

Just like if you slap a man while he's chewing tobacco you'll get some of the dirty stuff onto yourself too