'alkali-proof' originates from English by combining 'alkali' and 'proof'. 'alkali' ultimately comes from Arabic 'al-qaly' (or 'al-qalyī'), meaning 'ashes of the saltwort' (plant ashes), and the element 'proof' is from Old English/Old French roots meaning 'test' or 'resistant'.

'alkali' passed from Arabic 'al-qaly' into Medieval Latin/Spanish as 'alchali'/'alcalis' and then into Middle English as 'alkali'. 'proof' came via Old French (e.g. 'preuve') from Latin 'probare' (to test) and Old English reflexes, and in Modern English developed the sense 'resistant to' in compounds (e.g. 'water-proof', 'fire-proof'), yielding the compound 'alkali-proof'.

Initially, 'alkali' named a substance derived from plant ashes and 'proof' meant 'tested' or 'proven'; over time, in compounds 'proof' shifted to mean 'resistant to' or 'not affected by', so 'alkali-proof' now means 'resistant to alkali'.