Short for methylated spirits. Useful for stripping paint etc., but not to be taken recreationally. Gives rise to the following joke:

A tramp walks into a hardware store.
Tramp: Can I have a bottle of meths, please?
Shopkeeper: Go on, clear off. I'm not going to sell you meths, you're only going to drink it.
The tramp is offended.
Tramp: That's just bloody typical, that is. Just because I look a little down on my luck, you automatically assume I'm some sort of degenerate. As it happens, I've finally managed to pick myself up, I've sorted out a painting job, so I need a bottle of meths, but all I get is abuse!
The shopkeeper is abashed
Shopkeeper: Look, calm down, I'm sorry, I didn't mean anything by it. Hang on there and I'll get you a bottle.
Tramp: Thanks. Oh, and...
Shopkeeper: What?
Tramp: Make it a cold one?