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.