What does "sitting pretty" mean?

In an advantageous position; also, financially well off. For example, The terms of the will left Mary sitting pretty. Although the use of pretty in the sense of “advantageous” is much older, this colloquialism dates only from the early 1900s. It was given extra currency as the title of two different musicals, Sittin' Pretty (M. Moore, 1921) and Sitting Pretty (G. Bolton and P.G. Wodehouse, 1924).