(4 syl.). The Greek form of the Egyptian god Har-pi-kruti (Horus the Child), made by the Greeks and Romans the god of silence. This arose from a pure misapprehension. It is an Egyptian god, and was represented with its “finger on its mouth,” to indicate youth, but the Greeks thought it was a symbol of silence.
“I assured my mistress she might make herself perfectly easy on that score [his mentioning a certain matter to anyone], for I was the Harpocrates of trusty valets.” —Gil Blas, iv. 2 (1715).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894