Covering the Face
No malefactor was allowed, in ancient Persia, to look upon a king. So, in Esther vii. 5, when Haman fell into disgrace, being seen on the queen's divan, “they instantly cover Haman's face,” that he might not look on the face of Ahasuerus.
In India a low caste man covers his mouth when speaking to one of high caste.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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