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lubber's line

or lubber line

noun Navigation.
  1. a vertical line on the forward inner side of the bowl of a fixed compass, used as a reference mark indicating the heading of a vessel.
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Origin of lubber's line

First recorded in 1855–65
Also called lubber's mark, lubber's point.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018