Bitty seems to latch on to certain phrases. We have a couple of flags hanging on the wall in our garage and he told me once, "da fwag iss stuck." I replied, "it's supposed to be stuck" (knowing he wanted me to "unstick" it by taking it down and not wanting to do that). For weeks on end after that, he commented on EVERYTHING that was "supposed to be stuck."
"Da bear iss stuck, iss suppoh-ta-bee stuck." (the bear on his blanket)
"Da circle iss stuck, iss suppoh-ta-bee stuck." (the door-stopper on the wall at the doctor's office)
"Da cat iss stuck, iss suppoh-ta-bee stuck." (the cat-shaped cookie jar that I purposely put out of his reach)
And my favorite "Da Mama iss stuck!" (when he was trying unsuccessfully to push me out of bed to get him some chocolate milk). My reply? "Iss supposed to be stuck!" haha
I think this is a form of language "practice," testing to see what other situations he can apply his favorite phrases to. He can usually figure out from our responses (granted sometimes exaggerated for his benefit) whether he used a particular phrase appropriately or not, and then he'll adapt his use of it in the future.
This particular phrase seems to have faded. Maybe he managed to categorize pretty much everything in his environment now on the basis of whether or not it's "supposed to be stuck" and so has moved on?