Entry 2 senses Webster, 1913 Whereby /wâr-bī'/ · Where·by · IPA /wɛə(ɹ)ˈbaɪ/ 01 adv. By which; -- used relatively. 1. By which; -- used relatively. 2. By what; how; -- used interrogatively. “Whereby shall I know this?” — Luke i. 18.