What's the meaning of the phrase 'Sitting pretty'?
Comfortably placed or well situated.
What's the origin of the phrase 'Sitting pretty'?
We don't know how the expression 'sitting pretty' was derived. That is, we don't know who or what was being referred to as 'pretty' when the term was coined.
The image of a young woman posing for a photograph or ducks sitting tight and making a difficult target for shooters may spring to mind. The expression isn't known until the early 20th century so both of the above are plausible.
We do have something that can help locate the source of the phrase. The word 'pretty' has another meaning, in addition to the more usual 'attractive' and that is 'in an advantageous or safe position'.
The earliest uses of the phrase in print seem to point to 'sitting pretty' just meaning 'sitting (in a real or metaphorical sense) in a position of advantage.
Those first uses are all American and we can conclude that 'sitting pretty' was coined in the USA. Here's an example, from the Nebraska newspaper The Lincoln Star, May 1915:
Clyde Wares returned to his club in the metropolis yesterday and according to reports everything is ‘sitting pretty’.