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general::
Once involved, one must not stop at half-measures. For example, All right, I'll drive you all the way there? in for a penny, in for a pound. This term originally meant that if one owes a penny one might as well owe a pound, and came into American use without changing the British monetary unit to dollar. [Late 1600s] For a synonym, see HANGED FOR A SHEEP.
American Heritage Idioms