Thoroughgoing means complete, total, or extending to every part of something. When you do a thoroughgoing cleaning of your room, you vacuum under the bed, organize every drawer, and clean behind the furniture. Everything gets addressed, with nothing skipped.

A thoroughgoing investigation examines every detail and follows every lead. A thoroughgoing revision of an essay means rewriting and improving it from start to finish, addressing every section with the same level of care and attention.

You might call someone a thoroughgoing optimist if they stay positive in every situation, or a thoroughgoing professional if they handle every aspect of their work with care and competence. The word emphasizes that something isn't partial or halfway. It's complete in every respect. When a scientist conducts a thoroughgoing analysis of their data, they examine it from every angle, checking and rechecking their conclusions to make sure they haven't missed anything important.