Hereby means by this act, in formal declarations. I hereby declare, hereby resign, hereby give notice.
It marks the sentence as doing the thing rather than describing it — saying I hereby resign is the resignation, not a report of one. Philosophers call these performative utterances. It belongs to legal and ceremonial language only. Note here plus by.
1adverbherebyBy this act, in formal declarations.
“I hereby declare the new bridge open.”
hereby is a formal marker.
Here plus by, one of a family of legal compounds with hereto, herewith and hereinafter.
From here + by.
Etymology from Wiktionary, CC BY-SA