Interrogate is the Latin top of the ladder ask, question, interrogate. Ask is native Old English, question came through French, and interrogate was taken from Latin in the late 1400s — three words for the same act, sorted by formality, with the Latin one now belonging almost entirely to police stations and courtrooms.
The Latin is interrogare, from inter, "between," plus rogare, "to ask." Rogare seems to be a figurative use of a verb that literally meant "to stretch out the hand," from a root meaning to move in a straight line — the same root behind regular, direct and royal. Asking, on this account, began as reaching. English had also taken the French version of the word as interroge in the late 1400s, but that died, leaving the Latin form in sole possession.
interrogate (v.) "to examine by asking questions," late 15c., a back-formation from interrogation or else from Latin interrogatus , past participle of interrogare "to ask, question, inquire; interrogate judicially, cross-examine," from inter "between" (see inter- ) + rogare "to ask, to question," apparently a figurative use of a PIE verb meaning literally "to stretch out (the hand)," from root *reg- "move in a straight line." The Old French word was interroger (14c.) which yielded English interroge (late 15c.), now obsolete. Related: Interrogated ; interrogating . also from late 15c.
interrogate came in through Latin, and its spelling follows that family's conventions.
On the register ladder
English kept all three words for this idea, one from each layer, and the rung is itself the cue: the native word is blunt and concrete, the French one polite, the Latin one formal or technical.
| Old English | French | Latin |
|---|---|---|
| ask | question | interrogate |