Green’s Dictionary of Slang

1. (US) a toady [abbr. ass-kisser n.].

1939		‘Citadel Gloss.’ in AS XIV:1 Feb. 24/1: A.K. v. to curry favor with a superior. n. a cadet who indulges in the practice.
1971		E.E. Landy Underground Dict. (1972).
1989		H. Rawson Dict. of Invective (1991) 26: ass-kisser. A sycophant, a brown-noser; often abbreviated A.K.

2. sycophancy, behaving in a sycophantic manner [abbr. ass-kissing n.].

2000

S. Backauskas ‘How To Kiss A** Without Losing Your Soul!’ 🌐 One of the other negatives to AK is the fact that you need to deny how you feel in order to continue doing it. My friend Gretchen is the Queen of AK. I’ve watched her numerous times stroke clients, allowing them to make callous or inappropriate remarks, while maintaining a gritted-tooth grin.