The phrase "hollow men" occurs in JULIUS CAESAR IV, ii, 23-27, where it is used by Brutus when he learns that his former ally and fellow-conspirator, Cassius
Brutus reflects:
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But hollow men, like horses hot at hand,
Make gallant show and promise of their mettle;
But when they should endure the bloody spur,
They fail their crests, and, like deceitful jades,
Sink in the trial.
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is now behaving in a less friendly way towards him.