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2.
A person with whom one shares a rental property.

AKA a room mate or roomie.

Someone to split the rent with. (If they have the money)
Tom: Whoa, Somebody broke in and trashed your apartment while you were gone!
You: I wish. No, it was just my Flat mate, he's a pig.

You: Hey will you wash the dishes?
Flat mate: Ignores you and goes to his room.
by Pyro ND June 04, 2009
 
1.
1. A femail friend with small or non-existant mammaries
2. A person who lives in the same flat as you.
1.
What do you think of Emma?
She's flatmate
2.
I'm going for a beer with my flatmates.
by Fraser Steen October 20, 2006
 
3.
(Noun) Friends that aren't so well endowed/have small breasts, relatively female.
I'm just hanging out with my flat mates
by Syde Trak August 12, 2011
 
4.
I girl from Australia or New Zealand chick that is flat chested slut who you have sex with..
A Flatmate is; A flat chested girl from the the south pacific, generalized towards Kiwis and Aussies chick, whom were not "blessed" with tits!
by Dempset January 24, 2011