What does "how come" social English phrase mean?

It means "why". *How come you are here instead of at work? = Why are you here instead of at work?* *I'm going to throw this cereal in the trash.* *How come? = Why?* *Because nobody likes it and it's going stale.*

"How come" has the same meaning as why, but it's used slightly differently. You might use it to express surprise or indignation, when you would normally expect a different outcome than what actually occurred.

As others have said it means why. Its a contraction of the phrase: "How has it come to pass that..." Which sounds very formal and old fashioned

I have another question? Is it considered ruder than why? I used it in place of why in a sentence and was told my phrasing was rude.

Think of it as a short version of "how did things come to be this way?" or "How did this come to pass?"

It means "why"

[Example: How come people post here instead of just using a dictionary?](https://dictionary.cambridge.org/us/dictionary/english/how-come)

It's asking for an explanation of what you just said. "I can't come to dinner tonight after all." "How come?" "I have to collect my brother from the airport"

It means “why.”