Brutish and foolish. In fact, the original meaning of the idolatrous religions had begun, probably, to fade, and the worship of Bel and Nebo had become (as the worship of the Egyptian gods became at a later period) increasingly formal and ritualistic.
The stock is a doctrine of vanities; rather, an instruction of vanities; i.e. all that the idols can teach is vanities. Against this is the plural ("vanities," not vanity); it is more natural (and also more in accordance with usage; comp.
Genesis 41:26, Hebrew) to render, the instruction of the vanities is wooden ("vanities" has the constant technical sense of "idols;" see Jeremiah 8:19; Jeremiah 14:22; Deuteronomy 32:21; Psalms 31:6).
The clause then furnishes a reason for the folly of the heathen; how should they attain to more than a "wooden" knowledge, when the idols themselves are but wood? A bitter truth in an ironical form.