sink, subside, or bulge downwards under weight or pressure or through lack of strength

she let her head sag lower and lower

the bed sagged in the middle

decline to a lower level, usually temporarily

exports are forging ahead while home sales sag

sinking or bulging downwards under weight or pressure or through lack of strength

sagging shelves bearing rusty paint tins

becoming weaker; declining

a comprehensive overhaul meant to revive sagging sales