'convalescent' originates from Latin, specifically the word 'convalescens' (the present participle of 'convalescere'), where 'con-' meant 'together/completely' and 'valescere' meant 'to grow strong.'

'convalescens' passed through Late Latin/Medieval Latin usage and entered English (via learned borrowing) as the adjective and noun 'convalescent', keeping much of its original form.

Initially it meant 'becoming strong again' or 'growing strong together'; over time it evolved into the current meanings referring to someone recovering strength after illness (as a noun) and to the recovery period or state (as an adjective).