What does "a worthless doctrine" mean in Hebrew, as found in Jeremiah 10:8?
"But they are altogether dull-hearted and foolish; A wooden idol is a worthless doctrine." - Jeremiah 10:8 (NKJV)
The NKJV's translation 'a worthless doctrine' is derived from the Hebrew phrase above,
musar | ha-balim | ets | hu
instruction | of vapor | wood | he
The original KJV scholars aimed to to align their translation of words as best they can, and the word 'hebel' (הָ֫בֶל) which becomes ha-balim is not an easy word to get into English. It's the word which is well known for its usage throughout Ecclesiastes, and so this was (most likely) their anchor for translation of the word throughout the OT. The KJV thus reads:
10:8 "But they are altogether brutish and foolish: the stock is a doctrine of vanities." - KJV
And the New King James has taken the 'stock/wood' as a reference to idols, but kept the sense of the KJV text which they were working from. But where did the text get its idea of a 'doctrine of vanities' to begin with?
The KJV translators were more fond of the Septuagint (Greek) than the Vulgate (Latin) in guiding their translation, but unfortunately this verse from Jeremiah is missing from the Septuagint, so the Vulgate was most likely the best early guide they had to go by. In translating 'hebel', they chose to follow the translation of the Vulgate text, which had rendered this word consistently as 'vanitas':
10:8 "Pariter insipientes et fatui probabuntur: doctrina vanitatis eorum lignum est." - Vulgate
It is worth noting that vanitas comes from the Latin word vanus which is closer to 'empty' than 'vain' in the modern English sense. As our language has grown and changed, the idea of 'vanity' no longer matches the Latin sense as originally captured by the Vulgate. Many modern translations follow the Vulgate and KJV in their translation of this word, taking the English word 'vanity' as a direct equivalent of 'vanitas'.
Though the Septuagint does not have this passage from Jeremiah, it does translate 'hebel' throughout Ecclesiastes as ματαιότης, which comes from the root matan (μάτην), meaning pointless or 'in vain'. This again is not even using the English word 'vain' in the sense of vanity, but rather in the sense of fruitlessness.
'Vanity' is not a direct modern English equivalent of the Hebrew or Latin word, however, and so I prefer to take 'hebel' back to its root of 'vapor' (see Strong's 1892). This word captures a fuller idea of emptiness, worthlessness and ephemerality, rather than just "meaningless/vanity". This makes more sense in our text here as well as throughout Ecclesiastes.
In the text then, our phrase then becomes closer to 'instruction of emptiness, wood', which is taken as a reference to idols. A better English rendition might follow the Holman Christian Standard Bible:
10:8 "They are both stupid and foolish, instructed by worthless idols made of wood!" - HCSB
The emphasis seems to be that the teaching is taken from the wood/idols, rather than the teaching being of the wood/idols, though the Hebrew phrase could capture either idea. The verse also has a parallel in v14 which confirms the idea of 'breathless idols':
10:14 "Everyone is stupid and ignorant. Every goldsmith is put to shame by his carved image, for his cast images are a lie; there is no breath in them." - HCSB
For more discussion on the translation of hebel see this similar question: "What translation best translates the word 'vanity' in Ecclesiastes?"
The "worthless doctrine" the KJV renders for Jeremiah 10:8 is equivalent to talking about "empty teaching" which the Israelites of Jeremiah's time were taking from idols and idolatrous teachers.
The advantage of "vanity" as a translation of hebel is that it seems to be the only English word that communicates the sense of both "worthlessness" and "pride".
From Jeremiah's point of view the stock is a joke, because he knows the true God cannot be captured or represented in such a way, and indeed He commanded that it never be done. However, from the point of view of the stock-worshippers, it is a thing of pride. After all, they have the silver and gold, and they have the skilled artisans to craft their gods, and they have the fine go-to-church apparel (Jeremiah 10:9) -- all the trappings, but no substance.
This is precisely what has caused the people of Israel to stray -- they don't share Jeremiah's view that the stock of the heathen is a joke. They are drawn to it because their eyes are on the trappings, so they too, want to seek out and make inquiry of the stock. And here is the heart-ache of God: "If you would only trust Me", says Yahweh, "Then time would reveal the rock on which you've been established, and you would see for yourselves the worthlessness of the stock in which the heathen take such pride."
The KJV's translation of musar ha-balim as a "doctrine of vanities" is by far the best -- any teaching that would puff one up without reason. Jesus gives a great example of a doctrine of vanities in Luke 12:13-21, which is the essence of what the Preacher is on about in Ecclesiastes.
Any word that does not depict both "worthlessness" and "pride" cannot adequately communicate the proper sense of the Hebrew, hebel.