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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.