macasamhail.

From Middle Irish maccasamla. Apparently a phrase meaning literally “child of likeness” (macc (“son, child”) + samla, the genitive of samail (“likeness”)), though Marstrander suggests that the first element is actually Old Norse maki (“match”). This suggestion explains the presence of the second a; the “child of likeness” interpretation would then be folk etymology, triggering the loss of that a in some forms. That it was originally a phrase rather than a genuine compound is shown by the fact that the second element is in the genitive and by the lack of lenition on the s. It was later reinterpreted as a compound, with the second element samhail in the nominative from the mid-17th century onward. The form macshamail, with lenition of the second element, is rare and late, appearing in the Historical Irish Corpus no earlier than the 20th century, and is probably a hypercorrection. Cognate with Manx mac-soylley and Scottish Gaelic mac-samhladh.