to deviate or wander away from the main topic or purpose in speaking or writing; depart from the principal line of argument, plot, study, etc.
2.
Archaic. to turn aside.
Origin: 1520–30; < Latin dīgressus, past participle of dīgredī to go off, depart, digress, equivalent to dī-di-2 + -gredī, combining form of gradī to go; compare grade