I'm often accosted by a customer who says something akin to, "I just had this machine in for repair, and now it won't (multiple gripes follow)... "Just" seems such an ambiguous word that I can't imagine what the person might mean, although the implication is that I've done him wrong. Often "just" turns out to be last year, after a dozen tankfulls of water and/or dirt-laden fuel. Is "just" an UN-just word in your world, as it is in mine?

It's often verbal filler, like "er" and "um" and "basically". It can be used as a time marker (I've just done it) but, when it means anything, it's more commonly used to indicate an attitude on the part of the speaker that the task referred to is, was or should have been a trivial one. It can also express irritation (I'm just coming!, Now just hang on a minute.) with the other person.

Like all these little and apparently meaningless phrases what it means has more to do with social conventions and expectations than with any easily pinned down dictionary definition.

Just like most common words, "just" has many meanings. You just need to know when it's appropriate to use "just" and when it's better just to leave it out.

As Bob said, it's just a matter of social convention and just as many people like to use "just" as would prefer just to find an alternative.