From Longman Dictionary of Contemporary Englishbutter somebody ↔ up phrasal verbinformalPRAISE to say nice things to someone so that they will do what you wantDon’t think you can butter me up that easily. →butter→ See Verb table
Examples from the Corpus
butter up• From the title I expected Nathan to just butter Jackie up.• Agree with him. Butter him up a bit.• Silly-Willie popped them back in the oven and buttered them up himself.