Where does “rationale” come from?

Rationale comes from Latin rationale, derived from Latin rationalis, combining Latin reor meaning "to think or reckon" with Latin -alis, ultimately from Proto-Indo-European h₂reh₁-.

rationale (English): An explanation of the basis or fundamental...

Definitions

An explanation of the basis or fundamental...

Ancestry of “rationale”, step by step

rationale traces back through two separate lines of ancestry.

via Latin rationale

Step	Language	Word	Meaning
1	Latin	rationale	nominative singular neuter of ratiōnālis;...
2	Latin	rationalis	accounts; rational, reasonable, of or possessing...
3	Latin	ratiō	reason, reasoning, explanation, ground, motive, rationality, rationale, purpose
4	Latin	-tiō	tion, -ation, -ing
5	Latin	dissertātiō	(spoken) dissertation, discourse, disquisition
6	Latin	dissertus	arranged, disposed; explained
7	Latin	disserere	second-person singular future passive indicative...
8	Latin	dis-	asunder, apart, in two; reversal, removal;...
9	Latin	calceus	shoe
10	Latin	calx	limestone; chalk; the finish line
11	Ancient Greek	χάλιξ	small stone, pebble; gravel, rubble

via Late Latin ratiōnāle

Step	Language	Word	Meaning
1	Late Latin	ratiōnāle	—

Every word from Ancient Greek χάλιξ →Every word from Latin -tiō →Every word from Latin dis- →