Merriam-Webster's Collegiate® Dictionary, 11th Edition
THOROUGH
I. preposition
Etymology: Middle English thorow, from Old English thurh, thuruh, preposition & adverb Date: before 12th century through ,
II. adverb Date: before 12th century through ,
III. adjective
Date: 15th century
carried through to completion ; exhaustive ,
2. marked by full detail , careful about detail ; painstaking , complete in all respects , having full mastery (as of an art) , thoroughly adverb thoroughness noun