[Middle English, from Old French (a) certes, perhaps from Latin ad certās or from Vulgar Latin *certānus, both from Latin certus, certain; see certain.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
archaic with certainty; truly
[C13: from Old French, ultimately from Latin certus certain]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
certainly; in truth.
[1200–50; Middle English < Old French phrase a certes < Latin *ā certīs, literally, from sure (things); see a-4, certain]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.