'unstrained' originates from Old English and the verb 'strain' which comes via Old French and ultimately from Latin; the prefix 'un-' originates from Old English 'un-' meaning 'not', and 'strain' comes from Latin 'stringere' (via Old French) where 'stringere' meant 'to draw tight'.

'unstrained' was formed in English by adding the Old English negative prefix 'un-' to the past participle/adjective 'strained' (Middle English forms from Old French 'estraindre' or similar and ultimately from Latin 'stringere'), producing the modern adjective 'unstrained'.

Initially it would have been understood as 'not drawn tight' (literal sense related to tension), but over time it has also taken on the figurative sense 'not forced; relaxed or natural'.