“I'll warn you - It's still pretty damp.”

You and a friend are at a swimming pool. Your friend is about to get out of the pool, but he doesn't have a towel. You have one, but it's wet. You say:

I'll warn you - It's still pretty damp.

I'll warn you - (sentence)

Use this phrase to let people know about a problem or weakness before they find it themselves. For example, if you're going to play on a softball team but you're not a good player, you can tell the other team members:

I'll warn you - I'm not very good.

You use this phrase so that people won't get disappointed or complain.

a damp towel

The word "damp" means "a little wet", but it's more specialized. It's only used with a few other words. Here are the main ones:

damp hair

damp air 

a damp cloth

the ground is damp

"Damp" also sounds a little literary, so you're more likely to read this word in a book than to hear it in daily conversation.

pretty (something)

"Pretty ___" is somewhere between "kind of " and "really". It basically means "a little more than I expected". Here are some other examples of sentences that use "pretty":

It's pretty catchy.

Wow, this is actually pretty good.