(1) A hull
ashore, supported only at her ends, will sag in the middle. (See also: Hogged)

(2) Usually in combination as in ‘sag away to leeward’, or ‘sag off, it
des­cribes a boat making rather too much Leeway when sailing on the wind.
Typical small family cruisers tend to sag away most badly when the sea is
rough enough to slow their forward progress.