gallows humor

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Humorous treatment of a grave or dire situation: "conveying with gallows humor the utter insanity of the nuclear-arms race” ( New York).

Wiktionary

1. n. idiomatic Comedy that still manages to be funny in the face of, and in response to, a horrible, deathly, tragic, dramatic, perfectly hopeless situation.