'downslope' originates from English, specifically a compound of the words 'down' and 'slope'. 'down' ultimately comes from Old English 'dūn' where 'dūn' meant 'hill' or 'downward', and 'slope' comes from Middle English 'slope' (from Old Norse/Old English roots) where the root meant 'slant' or 'incline'.

'downslope' developed in Modern English as a straightforward compound of the existing words 'down' + 'slope'; the separate elements 'down' and 'slope' existed in earlier stages of English (Old/Middle English) and were later combined to form the modern compound 'downslope'.

Initially the component words referred to 'down' (downward or hill) and 'slope' (a slanting surface); when combined as 'downslope' the meaning became the specific 'downward side of an incline' and has remained largely stable in that sense.