Is this a proverb? What does it mean and what is the origin?
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.