In a neat, fashionable way, or quickly and cleverly.
From smart (originally meaning 'to feel sharp pain,' later 'brisk or quick,' then 'fashionable and neat') + -ly (adverb marker). Multiple meanings coexist in the word.
Military uniforms are called 'smart uniforms' because 'smart' historically meant 'sharp' and 'stinging'—and a well-pressed uniform with sharp creases literally looks crisp and 'biting' to the eye!
Complete word intelligence in one call. Free tier — 50 lookups/day.