Emily Dickinson Lexicon

THOR-OUGH, a. [thur'ro; Sax. thurh; G. durch; D. door. In these languages, the word is a preposition; but as a preposition we write it through. See this word. It is evidently from the root of door, which signifies a passage, and the radix of the word signifies to pass.]

1. Literally, passing through or to the end; hence, complete; perfect; as, a thorough reformation; thorough work; a thorough translator; a thorough poet. Dryden.
2. Passing through; as, thorough lights in a home. Bacon.