Scottish slang for cold, as applied to the weather.
Example: "You'd better take your coat, Mathew, its absolutely baltic outside."
Use of this word seems to connote a particular kind of cold day; dark, overcast, probably raining, or more likely sleeting. However, extremely cold weather does not seem worthy of the title. One possible explanation for this is that, on a merely cold day (as against, say, two feet of snow), one may chance it and wear a normal jacket, instead of the more sensible thick warm overcoat. One will then feel the cold a heck of a lot more and thus complain about it.
An interesting point to note is that the word, by virtue of having hard B, T, and C consonants and easily shoutable vowels, has found applications as a mild expletive. For example, if you're standing waiting for a bus in the cold and rain, shouting "Its BALTIC!!!" can have mild theraputic benefits.
The word is presumably a reference to the Baltic Sea, a rather cold, nasty place.