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
v. shun the company of others; "The new student keeps to herself"
withhold information; "I kept your little secreto to myself all these years"