Albert Barnes Commentary Jeremiah 10:8

"But they are together brutish and foolish: the instruction of idols! it is but a stock." — Jeremiah 10:8 (ASV)

Brutish — (Jeremiah 10:21) and foolish. Theirs was the brutishness of men in a savage state, little better than mere animals: their folly that of stupidity.

The stock ... — Rather, the instruction of idols is a piece of wood. That is what they are themselves, and “ex nihilo nihil fit” (from nothingness, nothing is made).