A ghost. Macbeth says to the ghost of Banquo -
“Hence, horrible shadow! unreal mockery, hence!”
Shakespeare: Macbeth, iii. 4.
He would quarrel with his own shadow. He is so irritable that he would lose his temper on the merest trifle. (See Schlemihl.)
Gone to the bad for the shadow of an ass. Demosthenes says a young Athenian once hired an ass to Megara. The heat was so great and the road so exposed, that he alighted at midday to take shelter from the sun under the shadow of the poor beast. Scarcely was he seated when the owner passed by, and laid claim to the shadow, saying he let the ass to the traveller, but not the ass's shadow. After fighting for a time, they agreed to settle the matter in the law courts, and the suit lasted so long that both were ruined. “If you must quarrel, let it be for something better than the shadow of an ass.”
May your shadow never be less. When students have made certain progress in the black arts, they are compelled to run through a subterranean hall with the devil after them. If they run so fast that the devil can only catch their shadow, or part of it, they become firstrate magicians, but lose either all or part of their shadow. Therefore, the expression referred to above means, May you escape wholly and entirely from the clutches of the foul fiend.
A servant carnestly desireth the shadow (Job vii. 2)—the time of leaving off work. The people of the East measure time by the length of their shadow, and if you ask a man what o'clock it is, he will go into the sun, stand erect, and fixing his eye where his shadow terminates; will measure its length with his feet; having done so, he will tell you the hour correctly. A workman earnestly desires his shadow, which indicates the time of leaving off work.
(To). To follow about like a shadow. This is done by some person or persons appointed to watch the movements and keep au fait with the doings of suspicious characters.
“He [Jesus] was shadowed by spies, who were stirring up the crowd against Him.” —Longman's Magazine, 1891, p. 238.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894