(medicine) Initialism of abdominal aortic aneurysm. / (organic chemistry) Initialism of aromatic amino acid. / (military) Initialism of anti-aircraft artillery.

(US, historical, law) Initialism of Agricultural Adjustment Act. / (US, historical, law) Initialism of Agricultural Adjustment Administration. / (US) Initialism of American Anthropological Association.

(sports, Canada, US) The highest level of minor league baseball, often used by extension in other sports to indicate the highest level of minor league play in that sport as well.

Initialism of awaiting aircraft availability.

(US, military, slang, vulgar) Chipped beef on toast.

(idiomatic) To engage in self-improvement; to begin a good habit or shed a bad habit.

not applicable / not available / not assigned, no assignment / non-assessable / not attested / not approved

Initialism of no answer. / Initialism of nothing added.

A long time ago; at some time in the past (a traditional beginning of children's stories, especially fairy tales).

(simile, of two things) Very similar; a couple made for each other.