A nautical expression indicating a water depth of 6 fathoms (36 feet, 10.97m) as measured by a sounding line; "deep six" acquired its idiomatic definition from the fact that something thrown overboard at or greater than this depth would be difficult if not impossible to recover. Marks on a sounding line were traditionally placed at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. The "leadsman" called out the depth as he read it off the line. If the depth was at a mark he would call "by the mark" followed by the number, if the depth was between two marks, he would call "by the deep" followed by the estimated number. Six fathoms would therefore be reported as "by the deep six."
- (idiomatic) To discard, cancel, halt; to completely put an end to something.
- They had put many hundreds of hours into the project before it was deep-sixed by management.