D defs.my
Entry 2 senses Webster, 1913

Whereby

/wâr-bī'/ · Where·by · IPA /wɛə(ɹ)ˈbaɪ/
01 adv. By which; -- used relatively.
  1. 1.
    By which; -- used relatively.
  2. 2.
    By what; how; -- used interrogatively.
    Whereby shall I know this?” — Luke i. 18.