Perhaps the off-putting trendy phrase of the decade, one I don't ever recall hearing previously, is the perplexing full sentence "welcome in."

The suddenness with which I've regularly heard it, both locally (such as when entering a shop) and on the Internet is startling, particularly because the phrase itself has such a wrong feeling about it. Has the speaker just recently come to believe that "welcome" is – perhaps always has been – insufficient? Has there arisen some concern about confusion with "welcome out?"

Specifically, tagging it with "in" seems to imply that the speaker somehow feels this use of "welcome" to be a verb, as though the phrase's structure matches that of "come in," "go in," "move in." It's worth a thought, since "welcome to…" is established. But that seems to be considered an exclamation or special case: Oxford doesn't list it as a phrasal verb (like "come along") or a prepositional very (like "approve of"). "This is a new special case!" might be a defense, and yes, verbality itself is an open plain, but that doesn't negate the slightly bludgeoning feeling that "welcome to," with its something to which to be welcomed, is clear of.

"In," in phrases like "come in," is considered an adverb: the same cog found in "come softly." (A lot of words seemed to be called adverbs which are merely substitutable in a sentence with adverbs, so I'll call that debatable.) But there's no validation there for "welcome in," since "welcome softly" doesn't make sense; "welcome" still isn't a verb there. ("Welcome" is a verb elsewhere, of course, but it's something welcomers do, not the welcomee to whom "welcome in" is addressed.)

Perhaps, then, the speaker is conscious that "welcome in" lacks the grammaticality of "come in," – they know it feels wrong – but enjoys the idea of attempting to lend or force the grammaticality. That's the kind of phenomenon that can happen when cultural issues bother people so much that they take their feelings out on language itself. I'll support this is a better choice than bashing others with objects, so long as they're content to unwittingly inspire people everywhere to step back and contemplate the relative fundamentals of language as we're doing here.

(There's the possibility I'm wrong about this whole premise, and that the invokers are simply hearing and saying "well, come in" with stilted emphasis. But, somehow, I don't get the feeling many would care to enter the captioning contest which would serve to shed light on that, even if a decent prize were available.)