dad-

See also: Appendix:Variations of "dad"

Welsh

From Proto-Celtic *do-ate-,^[2] equivalent to dy- +‎ ad-. Cognate with Cornish das-.

dad-

undo, un-, dis-
Synonym: dat-

‎dad- + ‎pacio (“to pack”) → ‎dadbacio (“to unpack”)

‎dad- + ‎gwneud (“to do”) → ‎dad-wneud (“to undo”)

‎dad- + ‎llwytho (“to load”) → ‎dadlwytho (“to unload”)
re-
Synonym: dat-

‎dad- + ‎ennill (“to win, to earn”) → ‎dadennill (“to regain”)

Welsh mutation
radical	soft	nasal	aspirate
dad-	ddad-	nad-	unchanged
Note: Some of these forms may be hypothetical. Not every possible mutated form of every word actually occurs.

^ R. J. Thomas, G. A. Bevan, P. J. Donovan, A. Hawke et al., editors (1950–present), “dad-”, in Geiriadur Prifysgol Cymru Online (in Welsh), University of Wales Centre for Advanced Welsh & Celtic Studies
^ Morris Jones, John (1913) A Welsh Grammar, Historical and Comparative, Oxford: Clarendon Press, § 156 i 13