Used rhetorically to express surprise about some revelation. Sometimes used ironically or sarcastically. Wow, it turns out that skinny little Mike Thompson has an incredible baritone singing voice. Who knew? A: "Man, this class is tough." B: "Yeah, who knew computer engineering would be so complicated?" A: "Har har, very funny."
See also: who
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.