'stouthearted' is formed in English from the adjective 'stout' + the combining element '-hearted' (from 'heart'). 'stout' originates from Old French, specifically the word 'estout' (meaning 'proud, brave' / strong), where the root conveyed sturdiness or strength; 'heart' originates from Old English 'heorte', where it meant 'heart' (the organ and metaphorically the center of feeling).

'stouthearted' developed as a compound in Middle English (attested forms like 'stout-herted' or 'stout-harted') and gradually standardized into the modern adjective 'stouthearted'.

Initially it literally conveyed 'having a strong/brave heart' and over time the phrase has continued to be used figuratively to mean 'brave, resolute, or possessing inner strength'.