A deadpan, snarky response to an unbelievable series of events that is related to you by a friend or co-worker. Taken from the classic Lee/Kirby issue of THE AVENGERS where Captain America is found in the Arctic, having been frozen in a block of ice since the end of World War II.
After he is thawed out, Cap relates the following tale: "By some fantastic stroke of fate, I must have been frozen in an ice flow, and then found by some eskimos who thought I was a supernatural object. Then, all those years, being in a state of frozen suspended animation must've prevented me from aging."
The rational response to all that malarky? "We believe you, Captain America."
Frantic Guy: I'm sorry I'm late! I was riding my bike here & nearly got hit by a plane making an emergency landing! I freaked out & slammed into a fire hydrant, flying over the handlebars and right into the arms a visiting Sumo Wrestler!
You: We believe you, Captain America.