A cold fish
Definition 1 of 1
Pro Tip 1/3Colloquial Use
The phrase is informal and best used in casual conversation or creative writing, not in formal settings. 
When he didn't bother to greet us, I realized he was a bit of a cold fish.
Pro Tip 2/3Emotional Distance
Implies a lack of emotional warmth or interest in others' feelings, not just general unfriendliness. 
Even after the good news, she acted like a cold fish, barely smiling.
Pro Tip 3/3Subtle Negative
Using 'a cold fish' subtly communicates a negative judgment about someone's social behavior. 
The new manager is a cold fish; he never joins in on the team lunches.