digress

Etymology

From Latin digressum, past participle of digredi.

verb

1. (intransitive) To step or turn aside; to deviate; to swerve; especially, to turn aside from the main subject of attention, or course of argument, in writing or speaking.
   […] For I hadn't had so much fun since the day / my brother's dog Rover / got run over. / (Rover was killed by a Pontiac. And it was done with such grace and artistry that the witnesses awarded the driver both ears and the tail – but I digress.) 1959, “In Old Mexico”, Tom Lehrer (music)
2. (intransitive) To turn aside from the right path; to transgress; to offend.