This expression is an idiom. As such it could have a meaning that extends beyond the literal, word-for-word translation.
Literal meaning: ‘bool’ is a log. Etz is wood or a tree.
This Biblical idiom may be used literally when referring to a real log of wood. In the Bible, it is a term of mockery of idols. Isaiah hints about the substance the idols are made of:
“And with what is left of it he makes a god, his carved image; he falls down to it, and worships it, and prays to it, and says, Save me, for you are my god!”
Later in this chapter the prophet is more specific about this god’s constituent:
“And none considers in his heart, neither is there knowledge nor understanding to say, I have burned half of it in the fire; yes, also I have baked bread upon its embers; I have roasted meat, and eaten it; and shall I make what is left of it an abomination? Shall I fall down to worship a block of wood?”
Clearly a god made of a block of wood!
Out of the contrast between the ‘wisdom’ of this kind of god and the everlasting wisdom of the true God — the idiom is coined and a mocking name for a stupid, good-for-nothing person becomes ‘bool etz,’ a block of wood.