"Ass in sling" is a colloquial expression that refers to a state of trouble or distress. Antonyms for this phrase could include "comfortable," "carefree," "secure," and "relaxed." These words imply a sense of ease and tranquility, rather than the stress and worry associated with having one's "ass in sling." Other antonyms could include "problem-free," "smooth sailing," "uncomplicated," and "unburdened." By using these contrasting words, one can paint a picture of a carefree and happy existence, free from the troubles and concerns that often come with life.