godan verb (-su), intransitive verb
get a fever
Archaic intransitive verb. In modern Japanese, 熱が出る or 熱がある are used instead. This form is found in classical texts.
古典では「熱す」と書いて「ねす」と読ませ、熱が出ることを表した。
In classical texts, 熱す was read as ねす and meant 'to get a fever'.
Archaic form of 熱する (ねっする), from 熱 (netsu, 'fever') + verbal suffix す. The exact historical derivation is uncertain, but it is the older intransitive counterpart.