The female version of the phrase "Mac-daddy"
'Dude she was pickin' up on all the guys, being the mactress that she is'
A girl that ACTS like she is good in bed, only to find out she just lays there like a MATTRESS!
I was out with my new girlfriend last night, and she kept bragging about how good she was in bed and what she was going to do to me. I found out later that she was a mactress!