full well

From Middle English fol wel, fulwel, from Old English ful wel, equivalent to full +‎ well. Compare Old Norse all-vel (“full well”).

full well (not comparable)

1. Very well
   • 15th c., “[The Creation]”, in Wakefield Mystery Plays; Re-edited in George England, Alfred W. Pollard, editors, The Towneley Plays (Early English Text Society Extra Series; LXXI), London: […] Oxford University Press, 1897, →OCLC, page 5, lines 120–121:

     He is so fayre, withoutten les, / he semys full well to sytt on des.

     He is so fair, without any limit; his appearance shows well when he sits on the dais.

   • 1829, Charles Burroughs, Memoirs and select papers of Horace B. Morse, Miller and Brewster, pages 83–87:

     Besides, there is always a tendency—indeed an interest—to take on what one knows full well.

   • 2006. Nadia Yassine. Full Sails Ahead. Justice and Spirituality Publications. page 2.

     He knows full well that I don't like that club.

Full was formerly used as an adverb in English. This has largely been replaced by fully, but full well remains in common usage, almost always modifying the verb to know. It is sometimes corrected to fully well, though this may be seen as incorrect or a hypercorrection.

• for a fact

• Brians, Paul. Common Errors in English Usage. Williams, James Co. 2009.