plane

Plane is the adverb of planus, "level, flat", and the sense runs the way English "flatly" runs: level ground, then plain speaking, then a flat "absolutely". In an answer on its own it means "quite so", and Cicero uses it that way constantly.

The trap is that plane is not a milder word than omnino. It is an intensifier: plane falsum is not "fairly false" but "false through and through". The other trap is spelling. This adverb has nothing to do with English plane the aircraft or plane the tool, and the identical string plane is also the vocative and ablative of planus, so a bare plane in a sentence is not always the adverb. English plain, plainly and explain come from the same root.