In the post-credit audio-scene of Birds of Prey, Harley says Batman is and then it cuts to black. What was she supposed to say? Is it about his secret identity, as she also named her hyena Bruce?
According to Gamespot, the quote appears to be "Batman f--".
Now, there currently is an animated Harley Quinn series on TV, and its premiere featured a very NSFW joke:
"Why are you protecting a psychotic clown who treats you like garbage?' the Caped Crusader asks, giving voice to countless fans. However, Harley isn't ready to consider that question just yet, and instead shuts down Batman with,
(I'm protecting the adult content with spoiler markup:)
"Asks the guy who fucks bats."
So perhaps this is what she was going to say?
We don't know, but we're probably being trolled by Harley / Cathy Yan
As several articles have pointed out, audiences of superhero films often expect a mid-credit and/or post-credit scene which may tease a sequel or provide an additional ending to the main story.
Given that Harley is somewhat chaotic in character, and is breaking the fourth wall throughout the film, it would fit that she knew we'd wait until the end for a post-credits tease, and that anything related to Batman would excite us. It is probably funny to her that we leave the cinema disappointed.