Is "how come" slang?

Sample Conversation:

A: How are you?

B: I am mad.

A: How come?

I thought that how come was a logical word choice but upon speaking with somebody for whom English is a second language, I found that they did not understand the meaning. I am wondering if using how come in this manner is a form of slang.

For some reason saying "How come you are angry?" sounds like bad grammar.

I can imagine the use of "How come, when I talk to you, you are angry?" to be correct with that added phrase in the middle.

So my question is, is using how come as in the example conversational slang? And also is "How come you are angry?" proper English?

Solution 1:

Looking in COCA, how come is very common in American English. It's used to mean why, in questions, and occasionally in statements:

How come the reporters aren't asking that?
And that's how come this song "This Land Is Your Land" became known throughout America.

If we break down the places it appears, a pattern emerges: how come is almost never used where formal writing is demanded, as in academic writing, but is frequently used where informality is okay, as in fiction dialogue and speech:

           TOTAL    SPOKEN  FICTION  MAGAZINE  NEWSPAPER   ACADEMIC
HOW COME   2689     882     1318     283       164         42  

In American English, at least, how come? is informal, but probably not considered slang.

Solution 2:

I would guess that "How come?" is simply a shortening of "How does it come to be (that)?".

Some digging finds the quotation in the 16th century Foxe's Book of Martyrs XII

After this, Tyndale corrected the same New Testaments again, and caused them to be newly imprinted, so that they came thick and threefold over into England. When the bishop perceived that, he sent for Packington, and said to him, "How cometh this, that there are so many New Testaments abroad? You promised me that you would buy them all."

Solution 3:

So my question is, is using how come in the example conversational slang?

Yes, it is part of spoken speech, but very rarely in written speech.

And also, is "How come you are angry?" proper english?

No, "how come" is not proper English. It's colloquial, or informal. The actual phrase is "how came", as in:

How came you to be so angry?

But that sounds very archaic. Other possible origins are "How comes it to be..." or "How did it come about?"

The normal way to ask a question is "Why?"

A: How are you?
B: I am mad.
A: Why?
B: ...

Solution 4:

It's not even that unique. In German it's not uncommon to say and write: "Wie kommt es, dass..." Literally in English: "How comes it, that..." So, to my 'native german ears' the "how come" actually sounds proper. I remember that it just appeared/felt a little short, like incomplete (thus confusing or funny) to me, when I heard it the first time.