sag

(noun) a shape that sags. Synonyms: droop. Example: "There was a sag in the chair seat."

(verb) droop, sink, or settle from or as if from pressure or loss of tautness. Synonyms: droop, swag, flag.

(verb) cause to sag. Synonyms: sag down. Example: "The children sagged their bottoms down even more comfortably."