unfruitful: yielding none of the fruit that was due, whether on a tree or in a person's life, and so producing nothing of benefit
This adjective is the ordinary Greek noun for "fruit" (καρπός) with a prefix that cancels it, exactly the way English turns "fruitful" into "unfruitful." That shape does more than note an absence. Because the word names the missing thing, it always measures something against what it was supposed to produce. Everyday Greek used it of land that grew nothing, a lake with no fish, a marriage that produced no children, and a tree that failed to bear.
In the New Testament it is almost always a verdict on people rather than on plants, and what counts as the missing fruit shifts with the setting:
- a life visibly changed by the message it received
- practical good deeds that meet real needs
- words in worship that others can follow and be helped by
- any good at all coming out of what is done in secret
The one nearly literal use sets the tone for all the rest. In Jude 1:12 false teachers are called late autumn trees with no fruit on them, and the timing is the whole point: late autumn is harvest, the very season when fruit should be hanging. That is the note the word carries everywhere. It is not a neutral shortage but a failure exposed at the moment results were due, which is why it so often shades into a warning of judgment.
A near neighbor helps fix its edge. The word ἀργός, "idle," names doing nothing at all; this word names having nothing to show for what was done. Peter sets the two side by side in 2 Peter 1:8, moving from inactivity to the empty harvest that follows it.