Lewis & Short

Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.

dēmensum, i, n., v. demetior.

dē-mētĭor, mensus, 4, v. a., to measure out, to measure, as a whole (whereas dimetior is to measure the parts of a whole—very rare): ut verba verbis quasi demensa et paria respondeant, Cic. Or. 12, 38; so Quint. 5, 10, 124 (al. dimensis): vos meministis quot calendis petere demensum cibum, i. e. the stated allowance of slaves, Plaut. Stich. 1, 2, 3.
Hence, dēmensum, i, n., a measured allowance, ration of slaves: quod ille unciatim de demenso suo comparsit, Ter. Ph. 1, 1, 9; Spart. Hadr. 7 fin.; Inscr. Orell. 2849; cf. Donat. ad Ter. l. l.; Sen. Ep. 80; Hor. Ep. 1, 14, 40 Orelli.
In a comic transf.: nunc argumentum vobis demensum dabo, Non modio neque trimodio, verum ipso horreo, Plaut. Men. prol. 14.