In a way that holds someone's attention and provides enjoyment or amusement.
From 'entertaining' (present participle of entertain) plus '-ly' (adverbial suffix). Traces back to Old French 'entretenir' with the sense of keeping someone occupied in a pleasant manner.
This is how you describe a teacher who makes math class actually fun, or a storyteller who keeps you on the edge of your seat—they do things entertainingly!
Complete word intelligence in one call. Free tier — 50 lookups/day.