(on a cargo vessel) the uppermost deck officially considered to be watertight: used as the level from which the Plimsoll marks are measured.
Is It Tis Or ‘Tis?According to Google's Ngram Viewer, the contraction 'tis was a fan favorite in the early 1700s. At this time, it was likely used more often than it's.
Origin of freeboard deck
First recorded in 1950–55
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019