1
general::
Free, not obligated; also, not occupied. For example, I am not at liberty to tell you the whole story, or "I . . washed when there was a basin at liberty" (Charlotte Brontë, Jane Eyre, 1847). This idiom is often used in a negative context, as in the first example. [First half of 1800s]
American Heritage Idioms
2
general::
Phrase(s): at liberty
free; unrestrained. • The criminal was set at liberty by the judge. • You’re at liberty to go anywhere you wish. • I’m not at liberty to discuss the matter.
McGrawhill's American Idioms And Phrasal Verbs