The phrase “beside the mark” is very old.
In fact, it is so old that, in the original Greek, it had passed from a proverbial phrase into a single word which expressed its figurative sense.
That is, in the old Athenian contests, an archer who failed to hit the mark was said to be out of the lists or course; hence, beside the mark.
The same thing, though in English, was said of the English bowman, sometimes by variation, “far from the mark,” “wide of the mark,” “short of the mark,” “to miss the mark.”
The Greek single word, which may be represented by roman type as exagonion, had the figurative meaning, “irrelevant; not pertinent,” precisely the meaning we give to the English phrase.