'untrustworthy-looking' is a compound formed from the prefix 'un-' (from Old English, meaning 'not'), 'trustworthy' (from 'trust' + '-worthy'), and 'looking' (present participle of 'look'). 'trust' has roots in Old Norse 'traust' and Old English verbs related to faith and confidence; '-worthy' comes from Old English 'weorth' meaning 'worthy' or 'having value'; 'look' comes from Old English 'lōcian' meaning 'to look or appear.'

'trust' developed in Germanic languages (Old Norse 'traust') and entered Middle English; 'trustworthy' arose by combining 'trust' with the adjective-forming element '-worthy' in later English. 'looking' is the modern continuous form of Old English 'lōcian'. These parts were combined in modern English to create the descriptive compound 'untrustworthy-looking.'

Initially the elements individually meant 'not' (un-), 'confidence/faith' (trust), 'having value' (-worthy), and 'to direct one's gaze or appear' (look). Over time they combined to form a compound whose meaning is specifically 'appearing not to merit trust' rather than any separate earlier senses.