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 come

How is it that, as in How come you're not attending the conference? Sometimes how come follows a statement and asks the question "why" or "in what way," as in You're not going? How come? The related phrase how so? functions the same way, as in You say she's changed her mind-how so?How come is short for how did it come about that and dates from the mid-1800s; how so, short for how is it so or how is it that , dates from about 1300.
See also: come, how
The American Heritage® Dictionary of Idioms by Christine Ammer.

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

how come

Informal
How is it that; why: How come you're so late?
See also: come, how
American Heritage® Dictionary of the English Language, Fifth Edition.
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