It is said that the founder of this famous family was named Halfpenny, and lived in Dublin in the 18th century. Having prospered in business, he called himself Mr. Halpen. The family, still prospering, dropped the H, and added Mac (son of), making MacAlpen; and Kenny MacAlpen called himself Kenneth MacAlpin, the “descendant of a hundred kings.” True or not, the metamorphose is ingenious.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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