1610s, "to alter or enlarge (a writing) by inserting new material," from Latin interpolatus, past participle of interpolare "alter, freshen up, polish;" of writing, "falsify," from inter "among, between" (see inter-) + polare, which is related to polire "to smoothe, polish," from PIE root *pel- (5) "to thrust, strike, drive," the connecting notion being "to full cloth" [Watkins].
Sense evolved in Latin from "refurbish," to "alter appearance of," to "falsify (especially by adding new material)." Middle English had interpolen (early 15c.) in a similar sense. Related: Interpolated; interpolating.