WHEREBY
Etymology
Adverb
whereby (not comparable)
(interrogative, obsolete) By what, in which direction; how.
By which.
(nonstandard) Where, wherein, in which.
Usage notes
Use of whereby as a formal equivalent of where is nonstandard and is avoided by careful speakers and writers, who use where or in which instead. The term typically fails readability and comprehension review so it is generally avoided in published works. The term is also avoided by speakers as it makes it difficult to understand the message one is trying to communicate.
Source: Wiktionary
Where*by", adv.
1. By which; -- used relatively. "You take my life when you take the
means whereby I life." Shak.
2. By what; how; -- used interrogatively.
Whereby shall I know this Luke i. 18.
Source: Webster’s Unabridged Dictionary 1913 Edition