'cruelly_treat' is a phrase combining 'cruelly' (the adverbial form of 'cruel') and 'treat'. 'Cruel' originates from Old French 'cruel', ultimately from Latin 'crudēlis' (from 'crudus' meaning 'raw, rough'); the adverbial suffix '-ly' forms adverbs from adjectives. 'Treat' originates from Old French 'traitier' (or 'traiter') coming from Latin 'tractāre', a frequentative of 'trahere' meaning 'to draw or handle.'

'cruel' came into Middle English via Old French from Latin 'crudēlis', becoming English 'cruel'; the suffix '-ly' was used in English to form adverbs (cruel → cruelly). 'Treat' evolved from Old French 'traitier/traiter' and Medieval Latin forms of 'tractāre', through Middle English into modern English as 'treat'. The phrase 'cruelly treat' is a straightforward combination of these elements in Modern English.

Originally, elements meant 'raw/rough' (crudēlis) and 'to handle/draw' (trahere/tractāre). Over time, 'cruel' came to mean deliberately causing suffering, and 'treat' retained the sense of how one handles or behaves toward someone, so the combined phrase now means 'to handle someone in a deliberately harmful or harsh way.'