What it means
Baltic means absolutely freezing. Not just a bit cold, proper can't-feel-your-fingers, breath-coming-out-angry freezing. People use it for savage weather, icy rooms, bus stops with no shelter, or any moment the cold feels personally disrespectful. Very British, a bit dramatic, and usually said with a moan that's fully deserved.
Usage examples
"You fancy a smoke out back? No chance, it’s proper baltic tonight, my face went numb just walking to the corner shop for milk."
"Bring a proper coat, it's absolutely baltic out there and the bus stop has no shelter."
"The flat was baltic all winter because the landlord wouldn't fix the heating."
"It was baltic waiting for the Uber, mate. I couldn't feel my ears after two minutes."
"Turn the heating on, this kitchen's baltic and I'm not making tea in gloves."
Where it comes from
It borrows the Baltic, that grey northern sea whose winters are a byword for bone-deep cold. Calling the weather baltic just lifts the chill of those shores and drops it on a British bus stop, no geography lesson required, only the shiver and the moaning that comes with it.
