British slang, possibly even specifically London slang. Used to describe
a participant in the peculiarly 90s phenomenon of young women acting in
a similar way to young men when socialising: ie drinking, being raucous
and letching at members of the opposite sex. A female version of the
New Lad in other words, obviously derived from one who is both a geezer
and a bird. Most women I know who this has been applied to take it as a
compliment.

A geezer bird enjoys a good honest night down the pub, and is able to hold her own in a laddish conversation. Men tell her they like her because she is "up for it" - this is the highest compliment a man can give.

Of course, a proper geezer bird is simultaneously laddish and attractive.

Being told you are a geezer bird by a man is like being told:

"While I find you to be an attractive woman, I enjoy the fact that you are not too uptight to laugh when I ask whether you have had more pricks in you than a second-hand dartboard."