completely and absolutely (`good' is sometimes used informally for `thoroughly')
"he was soundly defeated"
"we beat him good"
In a thorough or complete manner.
"The detective thoroughly investigated every clue to solve the mystery."
In plain English: Thoroughly means doing something completely and without leaving anything out.
"She thoroughly cleaned the kitchen before her guests arrived."
This adverb is formed by adding the suffix "-ly" to the adjective thorough. It entered English as a straightforward grammatical construction to mean "in a thorough manner."