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Wiktionary
keep to oneself

vb. 1 (context idiomatic intransitive English) To purposely avoid interaction with others. 2 (context idiomatic transitive English) To refrain from telling or otherwise expressing.

WordNet
keep to oneself
  1. v. shun the company of others; "The new student keeps to herself"

  2. withhold information; "I kept your little secreto to myself all these years"