Spreading butter on something; or figuratively, trying to gain favor with someone through flattery.
From the noun 'butter' (from Latin 'butyrum' from Greek 'boutyron'). The verb form meaning to spread butter developed naturally, and the figurative meaning (buttering someone up) emerged in the 1600s. Both meanings are common in modern English.
When someone's 'buttering you up,' you're being treated like toast—and this food metaphor shows how older languages compared flattery to coating things with a smooth, appealing layer!
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