"Naked" (arummim) — the near-homonym this verse's "subtle" (arum) is playing against.
Revelation 12:9 finally names this serpent explicitly - "the devil and Satan" - the figure left unidentified in Eden getting his name at the Bible's close.
Now the serpent was more subtle than any animal of the field which Yahweh God had made. He said to the woman, “Has God really said, ‘You shall not eat of any tree of the garden?’”
The man and his wife were both naked, and they were not ashamed.
The great dragon was thrown down, the old serpent, he who is called the devil and Satan, the deceiver of the whole world. He was thrown down to the earth, and his angels were thrown down with him.
The serpent's opening move isn't a lie — it's a question: "Did God really say...?" Up to this point in the story, God alone has defined what counts as good. The question doesn't attack that arrangement directly; it just quietly opens the possibility that the humans might be allowed to check God's definition against their own judgment. That's the whole temptation in miniature, before a single bite is taken.