To beat this particular flag to shreds:

Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.

half-mast, half-staff

To honor someone who has died, a flag is flown at half-staff everywhere except on shipboard, where half-mast is the term, according to naval usage, at least. Nevertheless either half-mast or half-staff is Standard these days to describe any such recognition on land or sea.