It means to be slightly wrong, or to be mostly right about something-- except that you've screwed up one part of it.
The origin is in theater technique, where every actor in a scene is supposed to move through the stageset according to a rigidly-timed scenario, and "hit his mark" on the stage at certain crucial moments. If he has lines, and the character he's talking to is on her mark, but he isn't on his, he may perform the part well, but because of other factors, the audience is confused about where his dialogue fits in the general scheme of things, or even who he's talking to. Meanwhile the actress can't always leave her mark and position herself in a way that makes sense-- the next bit of action that involves her requires her to be in the proper place, and if she's strayed away...
Well, in a group effort, if one person is off the mark, things soon become more and more disjointed-- in some plots the timing of interdependent actions is like clockwork, and a small error can escalate.
By analogy, this expression can be applied to any situation where careful prior planning or orchestration or choreography is not followed-- there are obvious military implications, for example.
In general it can mean "your aim is wrong," and in this sense the phrase is used when one's logic or argument is faulty. A physical mark on a stage is compared analogously to a "point" made in a series of argumentative issues-- which is to say, points.
I'm starting to sound like Polonius. Excuse me, I'm a little late finding my next mark-- I think it's right there behind the arras. Enter Hamlet, who gapes at me in disbelief that I'm still in plain view-- his upcoming dialogue with Gertrude makes no sense if I'm standing around listening to it, I'm supposed to be absconded behind the curtain eavesdropping. See what a tangent you've led me off onto, with all your questions about being off the mark!
I would like knowing about this expression as well, I have a vague idea so far, maybe I looked it up in a dictionary and it said : quick or slow off the mark quick or slow to begin, react to, or understand something and I read this expression in a book by Gerald A. Browne "19 Purchase Street" a man was realizing what type of guy he has been dealing, finally he was ripped off by this one and the sentence said.
"Costello found it hard to believe that could have been so off the mark with Windship (the guy who ripped him off) "
I think then that to be off the mark would means not to be aware (in this case) about the type of person you are dealing with.
I think this expression means : to be completely wrong, or mistaken
I have the following two references I found.
Rumsfeld critics are off the mark, the defense secretay’s opponents don’t understand the importance of creating a stable , elected government in Iraq. Christian Science Monitor
That’s a good guess, but slightly off the mark. In fact the carts contain just the amount of food that you eat in a year. Globe