'trustfulness' originates from English, specifically the adjective 'trustful' and the noun 'trust', where 'trust' meant 'belief, confidence' and the suffix '-ness' forms a noun indicating a state or quality.

'trustfulness' changed from Middle English forms such as 'trustfulnesse' (formed from 'trust' + '-ful' + '-nesse') and eventually became the modern English word 'trustfulness'.

Initially, related forms denoted simply 'the state of having trust or confidence'; over time 'trustfulness' also came to carry the nuance of 'readiness to trust (sometimes implying naivety)', while occasionally being used to mean 'trustworthiness'.