nidan verb (lower class) with 'yu' ending (archaic), intransitive verb
to partially heal; to get a bit better
Archaic intransitive verb meaning to recover partially from illness or injury. Found only in classical Japanese; not used in modern language.
The archaic word 'oroiyu' meant that an illness got a little better.
The exact derivation is uncertain; it is an archaic verb form that has fallen out of use.