My remarks on deep six came from J. E. Lighter, "Historical Dictionary of American Slang," vol. 1.

Webster's Collegiate Dictionary - "From "The leadsman's call 'by the deep six' for a depth corresponding to the sixth deep on a sounding line. 1944 slang: a place of disposal or abandonment- used especially in the phrase 'give it the deep six."
deep-six, vt (1952). 1. slang: to throw away: Discard 2 slang: to throw overboard."
Lighter is much better on dates of quotations than Webster's.

Like SRS, I thought everyone who spoke English knew the term deep six. I did have to look up 'deep,' OED)
Deep (Naut.) "A term used in estimating the fathoms intermediate to those indicated by marks on the sounding line, i. e. 1, 4, 6, 8, 9, 11, 12, 14, 16, 18, 19." The 6th deep is 11? Need a sailor, I'm lost here.
Usually pronounced 'dip.'