who are you, and what have you done with (someone)

who are you, and what have you done with (someone)

slang A humorous phrase emphasizing one's surprise at another's change in attitude or behavior, often for the better. You took out the trash before I even asked? Wow, who are you, and what have you done with my son? Look at these excellent grades! Who are you, and what have you done with Stephanie?
See also: and, done, have, what, who
Farlex Dictionary of Idioms.
See also:
  • who died and left you in charge
  • who died and made you boss
  • checks notes
  • contract
  • a verbal contract isn't worth the paper it's written on
  • under the thumb of (someone)
  • under (one's) thumb
  • under somebody's thumb
  • under someone’s thumb
  • under someone's thumb