How come?

how come

Why? Why is it that (something is the case)? A: "I'm sorry, but I'm not letting you go to the party tonight." B: "Aw, Mom! How come?" How come Bobby gets ice cream but I don't?
See also: come, how
Farlex Dictionary of Idioms.

How come?

How did that come about?; Why? Sally: I have to go to the doctor. Mary: How come? Sally: I'm sick, silly. John: I have to leave now. Bill: How come? John: I just have to, that's all. Henry: How come you always put your right shoe on first? Rachel: Do I have to have a reason for something like that?
See also: how
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

how ˈcome (...)?

(spoken) used to say you do not understand how something can happen and would like an explanation: ‘They’ve decided not to buy the house.’ ‘How come? I thought they definitely wanted it.’ If she spent five years in Paris, how come her French is so bad?
See also: how
Farlex Partner Idioms Dictionary
See also:
  • be a (something) to (one's) fingertips
  • be pressed for time
  • a run on (something)
  • be pushed for time
  • address (one's) comments to (someone or something)
  • address comments or remarks to
  • a rush on (something)
  • address (one's) remarks to (someone or something)
  • be not cut out for something
  • be in it for