putting on a brave front; pretending not to care
Idiomatic expression describing someone who acts nonchalant or defiant when they are actually in a difficult or shameful situation. The literal image is of a convict singing a carefree song while being led away. Used to comment on forced cheerfulness or false bravado.
Even though he lost, he was laughing as if he didn't care, putting on a brave front.
Her cheerful attitude is nothing more than a brave front.