Butter up
Definition 1 of 1
Pro Tip 1/3Social Dynamics
Use 'butter up' to describe strategic flattery in social interactions, especially to get what you want. 
He's trying to butter up his uncle to borrow the car.
Pro Tip 2/3Sincerity Questioned
When someone butters up another, it might imply the flattery isn’t genuine and has a motive. 
People can tell when you're just trying to butter them up.
Pro Tip 3/3Casual Context
The phrase 'butter up' is casual, more suitable for conversations than formal writing. 
I need to butter up my sister before I ask for a favor.