The "Never Ending Pie Throwing Robot" is a song Finn sings in the episode "What is Life?" while he builds Neptr, a robot designed to throw pies and prank Jake. The lyrics are describing how Finn thought he would prank Jake.
I'm gonna build me a pie throwin' robot
That throws never ending piii-ies
On Jake's ugly faaa-ace
And he won't know what hit him
'Cause the pies are
It's never ending pie throwing time!