Dog-cheap.
A perversion of the old English god-chepe (a good bargain). French, bon marché (good-cheap or bargain).
“The sack … . would have bought me lights as good-cheap at the dearest chandler’s in Europe.”—Shakespeare: 1 Henry IV., iii. 3.
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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.