'd'appalto' originates from Italian, specifically the word 'appalto', where 'appalto' developed from the verb 'appaltare' (Medieval/late Latin formation) meaning 'to entrust or assign under contract.'

'appalto' changed from Medieval Latin/vernacular verb forms such as 'appaltare' and became the Italian noun 'appalto' and the contracted prepositional phrase 'd'appalto' (originally 'di appalto').

Initially it referred to the act of entrusting or assigning (work) to someone; over time it evolved to refer generally to matters 'of a contract' or 'by contract/tender' in procurement and construction contexts.