OK, I've done a little research, and if you look here , you'll find lots of information about what Ch'ti means and where it comes from and the medieval era and blah blah blah.
That is, if you read French.
If not, I'll sum up quickly. It's a phrase that means essentially, "It's you" in a dialect called "Le Picard" related to French and Latin spoken in the area called the Nord-Pas-de-Calais. It appears that this region is known for mining and miners, and the dialect is spoken by them. It is also a beer-brewing area, thus the beer. (Which is, by the way, good.)
So "Ch'ti" means "It's you" and "Ch'mi" means "It's me." In modern French, we would say, "C'est toi" and "C'est moi."
(For non-French-speakers, those are pronounced "Say Mwa (just like Miss Piggy pronounced it)" and "Say Twa.")
And that ends today's bit of random trivia.