A way of saying "I should understand this completely, but it makes no sense whatsoever."
Origin: In the 1980s ski comedy "Hot Dog: The Movie," the antagonist proposes a Chinese downhill to resolve all conflicts between himself (the hotshot skier) and the protagonist (not so talented skier/cool guy). After they outline what's at stake -the girl, rule over the slopes, cash, etc. -the comic-relief Chinese guy leans in and asks "Wha te fuck is a Chinese downhir?" (A Chinese downhill is a ski race with a shotgun start -first one to the bottom wins.)
J1 -"Good luck getting Pee Wee Herman to discuss his movie theater fetish. You better get on it; deadline is Friday."
J2 -"Pee Wee Herman about what? When did this happen? What the fuck is a Chinese downhill?"