Basic Guidelines For English Spellings
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1Treatment with silence, rather than comment; the tactic of ignoring (an issue, accusation, etc.) instead of responding or reacting to it (now rare).
2(With the) the stubborn refusal to talk to or deal with a person, especially because of a recent argument or disagreement; chiefly in "to give (a person) the silent treatment", "to get the silent treatment".
Early 19th century; earliest use found in Edinburgh Journal Science.