Strange is in that awkward, dead tired mood where even the stupidest things are funny. As such, the idea of Ignatius hatching from an egg and getting egg gunk all over the place is kind of hilarious. It makes more sense than Strange's original hypothesis of the Ringmaster just puking Ignatius up...but it's still super silly.

He giggles a little and only makes a bare minimum to hide it.

"The only thing he told me was that he was 'unbirthed.'" Which based on Strange's tone of voice, he's got no clue what that word means. "You should ask Childermass about this. After all, he was there when Ignatius was on the moon. Maybe he saw a shell or something?"