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do not spoil the ship for a ha'porth of tar
Do not ruin something for an avoidable reason. "Ship" is thought to be a dialectical pronunciation of "sheep," and a "ha'porth" is a "halfpennyworth." Tar was used to protect sheep skin from flies (and thereby illness and death), so not having enough tar would contribute to the death of the sheep. You know your mom is going to be offended, so please call her before the dinner party—do not spoil the ship for a ha'porth of tar.
spoil the ship for a ha'porth of tarmainly BRITISH, OLD-FASHIONED
If you spoil the ship for a ha'porth of tar, you spoil a large or important piece of work completely because you refuse to spend a small amount of money on one aspect of it. I think it's a modest investment that is well worth making. You don't want to spoil the ship for a ha'porth of tar. Note: `Ship' in this expression was originally `sheep'. A `ha'porth' is a `halfpenny's worth'; a halfpenny was a British coin of very low value. Shepherds used to put tar on their sheep's wounds and sores to protect them from flies, and it would be foolish to risk the sheep's health in order to save a small amount of money.