Green’s Dictionary of Slang

butter v.

1. [late 17C–late 18C] of a gambler, to increase one’s wager; thus buttering adj.

2. [18C+] to flatter; thus to disguise with euphemism, flattery etc.; thus buttered, adj., flattered; buttering n., flattery.

3. [19C] to whip, to thrash.

4. [1990s+] (US campus) to like.

In derivatives

butterer (n.)

[mid-19C] a flatterer.

In phrases

butter up (v.) (also butter down, butter out)

1. [early 19C+] to flatter, to ingratiate oneself; thus buttering up, excessive flattery.

2. [mid-19C] of an inanimate object, to extol, to praise.

SE in slang uses

In phrases

butter the fish (v.) [ety. unknown; culinary imagery]

[1920s] to win at cards.