unthinking, senseless: failing to use the mind one has, so that what should be plain is never put together
The word is a negative prefix attached to the Greek verb for perceiving or understanding, so at root it means "not thinking, not taking it in." That is its whole force. It is not a measure of how clever someone is, and it is not first of all a verdict on their character. It names a mind that is not doing work it is perfectly able to do.
In the New Testament it lands most often as a rebuke, and the rebukes share a shape: the person being corrected already has everything needed to see the point. The risen Jesus says it to two disciples who knew the prophets and still could not put together what had just happened in Jerusalem (Luke 24:25). Paul says it to the Galatians, who had the crucified Christ set before their eyes as plainly as a public notice (Galatians 3:1), and says it again when they try to finish by human effort what God's Spirit began (Galatians 3:3). An expectation is built into the word: you are able to think better than this.
It is not always a rebuke, though. Paul can use it as a flat label for a group of people, the unreflecting or unschooled set beside the wise, with no sting attached (Romans 1:14). It can also describe things rather than people: the cravings of someone set on getting rich are called unthinking, not because they are silly, but because no sound reckoning stands behind them, and 1 Timothy 6:9 says they pull people under. And it can sum up a whole former life, as in Titus 3:3, where being without understanding sits between being disobedient and being led astray.
Greek has several words that English flattens into "foolish," and they are not interchangeable:
- ἄφρων is the fool of Israel's wisdom writings, the person who leaves God out of the reckoning altogether.
- μωρός is the blunt insult, branding someone dull and worthless, and is the word behind English "moron."
- ἀσύνετος names a failure to put things together, to see how the pieces connect.
This word is the mildest of that group and the most precise: the mind is there, and it is simply not being used.