"Arguing semantics" is a criticism utilized by folks who are either unfamiliar with, or fail to understand, the projects of Russell, Wittgenstein, the Vienna Circle, Logical Positivism, etc. Typically, when a person says "Todd is just arguing semantics", the person means that Todd is focused more on the linguistic articulation of a thing than ...

"'It's just semantics' is a common retort people use when arguing their point. What they mean is that their argument or opinion is more valid than the other person's. It's a way to be dismissive of language itself as carrier for ideas.

When it is encountered in general use today (among non-specialists) the word is often seen in the phrase just arguing semantics, which appears to indicate that the speaker intends for semantics to signify something unimportant and trivial, or unrelated to the discussion.

These aren't the cases where people are getting called out for arguing semantics. That's asking someone to expand on their argument, which is fine. The problem comes when people get caught up on the classification rather than in the meat of the argument.

It's a way of trying to shut the other person up when you don't have a valid response to make. Sometimes, it's a way of saying that what you said is the same thing that the other person (or a third party) said, in which case they're confusing semantics with synonymous.