'downward-slanting' is a modern compound formed from the adjective 'downward' and the present participle 'slanting'. 'downward' originates from Old English, specifically the elements 'dūn' (meaning 'down, hill') combined with the directional suffix '-weard' (meaning 'toward'). 'slant' originates from Middle English (e.g. 'slenten'), ultimately influenced by Old Norse and related Germanic roots meaning 'to lean' or 'to slope'.

'downward' changed from Old English forms such as 'dūnweard' into Middle English 'downward' and then the modern English 'downward'. 'slenten' in Middle English developed into the verb 'slant' in modern English; its present participle form 'slanting' has been used to describe inclines, producing the compound 'downward-slanting'.

Initially the components meant 'toward the down direction' and 'to lean/slope'; over time they combined into the current descriptive sense of 'sloping toward a lower position' and, by extension, a figurative sense of 'tending downward' or 'declining'.