To sink in the middle when supported at both ends; as a long pole.--Worcester. Provincial in England, but in common use with us.
·noun State of sinking or bending; sagging. II. Sag ·vt To cause to bend or give way; to <<Load>>. ...
Webster's Dictionary of the English Language
He begins to sag ; i. e. to decline in his health. Norf. and Suff. ...
A glossary of provincial and local words used in England by Francis Grose