Most hateful, as dearest foe. The word dear, meaning “beloved,” is the Saxon deor (dear, rare); but dear, “hateful,” is the Anglo-Saxon derian (to hurt), Scotch dere (to annoy).
Would I had met my dearest foe in heaven, Or ever I had seen that day, Horatio.
Shakespeare: Hamlet, i.2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894