'Perspire' meant 'breathe through the skin' — then became a genteel way to avoid saying 'sweat.'
To give out sweat through the pores of the skin as a result of heat, physical exertion, or stress.
From Latin perspīrāre (to breathe through, to blow constantly), composed of per- (through) + spīrāre (to breathe, to blow), from PIE *speys- (to blow). The original English medical sense, attested from the 1640s, was 'to breathe or exhale through the pores of the skin' — the idea that the body 'breathes' not only through the lungs but through the skin, and this cutaneous respiration is visible as moisture. The euphemistic use of perspire to mean 'to sweat' developed in the 18th century as part of genteel vocabulary reform: polite society
The word 'perspire' was adopted in the eighteenth century as a genteel euphemism for 'sweat.' The old joke — 'Horses sweat, men perspire, and ladies merely glow' — captures the class dynamics of the substitution. 'Sweat' was considered too coarse for polite company, so the Latin-derived 'perspire' provided a more refined alternative. The euphemistic use was so successful that many English speakers