APAD:in for a penny, in for a pound
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楼主 (文学城)
in for a penny, in for a pound
Etymology
Originally with reference to the fact that if one owed a penny, one might as well owe a pound (pound sterling, UK currency) as the penalties for non-payment were virtually identical in severity.
Proverb
in for a penny, in for a pound
Having started something, one must see it through to its end, rather than stopping short; one must “go the whole hog”.
1964, Sanki Ichikawa, The Kenkyusha Dictionary of Current English Idioms, page 509: …in for a penny, in for a pound: if one undertakes something, it must be carried through at whatever cost.