Used as a noun,one who does not come through with plans or an agreement. In its worst connotation, it may be used to describe someone who intentionally does something to sabotage you or screw you over.
Verb= to boscramble
"I can't believe he didn't finish the editing in time. He sure turned out to be a boscrambler!"
"What a boscrambler!"
"Even though you never come through, I still appreciate your boscrambling friendship."