adverb
efficiently; skillfully; deftly
Describes doing something with practiced skill, speed, and neatness; often implies a smooth, competent handling of a task.
彼女は手際良く書類を整理した。
She efficiently organized the documents.
料理を手際良く作り終えた。
I finished cooking skillfully and quickly.
Derived from the noun 手際 (てぎわ, 'skill, knack') combined with the adverbial form of 良い (よい, 'good'). The exact historical development is straightforward.