A promise, just like pie crust, that is easily made and easily broken.
Joe: Ay, you promised you would help me on my assignment
Chris: Dat was a pie crust promise so don't even trip.
A seemingly fragile or empty promise that is easily broken.
In the words of Mary Poppins: "No Pie Crust Promises. Easily Made, Easily Broken."
Joe: "Baby, I promise that as soon as I get out of jail, I'll get a job and take care of our baby."
Heather: "I've heard that one from you before. I aint believin' your pie crust promises no more."
A piecrust promise is a promise you make, but you never fulfill it. The piecrust part is like saying you just have the crust of a pie, not the yummy filling. This could be called an empty promise.
I won't make a piecrust promise to you!