I believe it's when you strike a drop target and it does not drop.
Edit: I have heard drop target rejects described as "brick" for YEARS! Especially my Catacomb drops.
Great to see you're enjoying Pinside! Did you know Pinside is able to run thanks to donations from our visitors? Please donate to Pinside, support the site and get anext to your username to show for it! Donate to Pinside