To make something taste sweeter by adding sugar or another sweet ingredient, or to make something more pleasant or acceptable.
From Old English 'swete' (sweet) plus the verb-forming suffix '-en.' The figurative meaning (making a situation more pleasant) arose naturally from the metaphor of improving taste improving overall quality.
Marketing uses 'sweetening the deal' constantly because our brains literally link sweet taste to pleasure and trust—studies show people are more cooperative after tasting sugar, so the metaphor isn't accidental; it's drawing on deep biology.
Complete word intelligence in one call. Free tier — 50 lookups/day.