to oneself
to oneself (1) {adv. phr.}
1. Silently; in the thoughts; without making a sign that others can see; secretly.
Tom thought to himself that he could win.
Mary said to herself that Joan was prettier than Ann.
Bill laughed to himself when John fell down.
2. Without telling others; in private; as a secret. — Used after "keep".
Mary keeps her affairs to herself.
John knew the answer to the problem, but he kept it to himself.
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Source: A Dictionary of American Idioms
