'sly-looking' originates from Modern English as a compound of 'sly' + present participle 'looking', where 'sly' meant 'cunning' and 'look' meant 'to direct one's gaze or appear.'

'sly' goes back to Old English (for example, Old English forms like 'slyġe'/'slyġ') and Proto-Germanic roots carrying the sense of 'cunning' or 'crafty'. 'Look' comes from Old English 'lōcian' meaning 'to look'. The compound 'sly-looking' is a modern English formation combining these elements to describe appearance.

Initially, 'sly' meant 'cunning' and 'look' meant 'to direct the eyes; appear'; over time the compound came to mean specifically 'having an appearance that suggests slyness' rather than describing behavior directly.