Thoroughly means completely and in full detail, missing nothing. Wash it thoroughly, thoroughly checked, thoroughly enjoyed it.
It has two jobs. With verbs of doing it means completely and carefully: investigate thoroughly, mix thoroughly. With adjectives and verbs of feeling it is simply an intensifier meaning entirely: thoroughly miserable, thoroughly deserved, thoroughly enjoyed. The second is much the commoner in speech. Note thorough, the adjective, which is pronounced THUR-uh in Britain and rhymes with nothing convenient.
1adverbthoroughlyCompletely and in full detail.
“The whole system was thoroughly checked and passed.”
2adverbentirelyEntirely, used for emphasis.
“We thoroughly enjoyed the concert and stayed until the very end.”
thoroughly is a degree of completeness.
Proto-Indo-European
*terh₂-
Old English
þuruh
Middle English
thoruȝ
English
thorough
English, today
thoroughly
From thorough + -ly. thorough comes from Middle English thoruȝ, þoruȝ, from Old English þuruh, a byform of Old English þurh, whence comes English through. The adjective derives from the preposition and adverb. The word developed a syllabic form in cases where the word was fully stressed: when it was used as an adverb, adjective, or noun, and less commonly when used as a preposition.
Etymology from Wiktionary, CC BY-SA