I’ll just be a minute!
This isn’t just! It’s not fair at all!
Here we can see two very different uses of the word just. How is it that we use it in such different ways?
The word just comes from the Latin jus, meaning law or right. From this we can clearly see how its use in the second sentence (as an adjective), meaning right or fair, came about.
The other, much more common use (as an adverb), meaning only, barely, or merely split off on a bit of a tangent from this meaning. In Middle English, just was used to mean exactly, precisely, or punctually (which we can still see today in phrases like just right). This isn’t too different from the sense of right. If something’s exactly how you want it, it’s right for you. Just right.
It’s not too difficult then to imagine how the meaning of just as exact gradually drifted from this to mean only or simply. If something is exact, it’s singular, the only way you want something. We can also use only (originally meaning one-like) to mean barely or merely (as barely and merely related to a quantity mean close to one).
I think that through this association between just and only, and then between only and barely/merely, led to the gradual association between just and merely/barely.
It’s just that simple!