whereof

Then again, Hice does have something of a history of mixing politics and religion, so at least he knows whereof he speaks.
Meet the Man Running for Congress on an Anti-Muslim Platform|Dean Obeidallah|July 24, 2014|DAILY BEAST
Moreover, as a former security chief he knows whereof he speaks.
Last Chance Saloon|Daniel Gavron|January 28, 2013|DAILY BEAST
In faith whereof, we, the respective Plenipotentiaries, have signed this treaty and have hereunto affixed our seals.
The Philippine Islands|John Foreman
The very moment he arrived I found myself again, I recognized my own existence, whereof I had not been conscious these ten years.
Marguerite|Anatole France

In witness whereof I have hereunto set my hand the day and year above written.
Putnam's Handy Law Book for the Layman|Albert Sidney Bolles
In witness whereof, the said party of the first part has hereunto set his hand and seal the day and year first above written.
Putnam's Handy Law Book for the Layman|Albert Sidney Bolles
In witness whereof, the parties to these presents have hereunto set their hands and seals.
Putnam's Handy Law Book for the Layman|Albert Sidney Bolles

British Dictionary definitions for whereof

whereof

/ (wɛərˈɒv) archaic, or formal /

adverb

of what or which person or thing?

pronoun

of which (person or thing)the man whereof I speak is no longer alive

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012