See digress in Webster's New World Roget's A-Z Thesaurus II
verb
To turn away from a prescribed course of action or conduct:
depart, deviate, diverge, stray, swerve, veer. (Archaic) err. See approach, correctTo turn aside, especially from the main subject in writing or speaking:
deviate, divagate, diverge, ramble, stray, wander. Idiom: go off at (or on) a tangent. See approach
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