'fainthearted' is a compound formed in English from 'faint' + 'hearted'. 'faint' ultimately comes from Old French 'feint' (from Latin 'fingere'), and 'heart' comes from Old English 'heorte'.

'faint' passed into Middle English with senses of weakness or lack of strength and 'hearted' (from 'heart') was combined to form the adjective 'faint-hearted' in Early Modern English; over time the hyphenated form became the single word 'fainthearted'.

Initially, 'faint' had senses related to weakness or being feeble (and earlier related to 'feigned' via Latin), but over time the compound came to mean specifically 'lacking courage' or 'timid', which is its current primary sense.