These other definitions are weird. A flatleaver is somebody who breaks plans with one or more persons without telling them either to hang out with some one else or just say at home instead of just saying no. A flatleaver is often the one who suggests hanging out, but can also be the other. Comes from the phrase to leave someone flat meaning high and dry and without warning.
"Me and Kay asked if I wanted to hang out today so I got all dressed and never heard from her until the next week asking the same thing, but never mentioning her not showing up last time...then did it again that week. She's a bitch and a flatleaver and if I see her again I'm gonna mob on her."