variant of ad- before p: appear.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
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
patronymic prefix in Welsh names, earlier map "son," cognate with Gaelic mac. Since 17c. merged into surnames (Ap Rhys = Price, Ap Evan = Bevan, etc.).
It is said that a Welshman who evidently was not willing to be surpassed in length of pedigree, when making out his genealogical tree, wrote near the middle of his long array of 'aps' -- "about this time Adam was born." ["Origin and Significance of our Names," "The Chautauquan," Oct. 1887-July 1888]
form of Latin ad- before words beginning in -p-; see ad-.
Online Etymology Dictionary, © 2010 Douglas Harper