rationaliserationalise

Rationalise comes from French rationaliser ‘to rationalize’, and goes back to Latin reor ‘to compute’.

1. Latinc. 75 BCE–200 CEreor¹‘to compute’
2. Latinc. 75 BCE–200 CEratiō²‘reason, reasoning, explanation, ground, motive, rationality, rationale, purpose’
3. Latinc. 75 BCE–200 CEratiōnālis³‘rational, reasonable, of or possessing reason’
4. Frenchc. 1600–presentrationnel⁴‘rational’
5. Frenchc. 1600–presentrationaliser⁵‘to rationalize’
6. Englishearliest quotation 2020rationalise⁵

1. ¹ Wiktionary, Latin ratio
2. ² Wiktionary, Latin rationalis
3. ³ Wiktionary, French rationnel
4. ⁴ Wiktionary, French rationaliser
5. ⁵ Wiktionary, English rationalise
6. Earliest quotation in Wiktionary’s entry: 2020, Gasworks Tunnel: “The eastern bore became disused in 1972-73, when its…”.

Earlier history not traced here. See Wiktionary.