That's not what you said.
In fact that was exactly what I said. You are misconstruing "... first said ..." to include an additional meaning of immutable.
What you said, exactly, was "Words mean what they mean
as they were first meant," [emphasis mine] quite clearly implying that the meaning thereof does not and can not change.
Had you at the time further related that comment to looking at a text (in context of this discussion, a song lyric) through the lens of Historical Criticism, then you would likely have not garnered the responses you received but you did not. Only your subsequent protestations suggest this is the case.
Finally, please kindly provide me an example of an English word that no longer means what it originally meant.
A cursory search will reveal numerous examples.