rectus

Rectus, recta, rectum is the perfect participle of rego, "to rule, to guide", used as an adjective. That is the whole word in one line: what has been ruled, in the draughtsman's sense of a ruled line. Both meanings hang off it, the physically straight and the morally straight, and Latin sees no metaphor in the jump. Comparative rectius, superlative rectissimus.

Two things to watch. The adverb is recte, "rightly, properly", not rectum; rectius, which the corpus supplies, is the comparative of that adverb as often as the neuter of the adjective, and only the sentence tells you which. And rectum as a neuter noun in Latin means "the right thing", not the anatomical one, which is a much later clipping of the medical intestinum rectum, "the straight gut". English takes correct, direct, rectify, rector and erect.