(Literally: "to rot off/away".)

This
verb is mainly used in its command form (imperative). In Dutch you
create the imperative by removing -en from the verb. However, you can
also use the infinitive as a command form, see ‘Examples’.

In the case of "oprotten" and similar verbs, one can also use
the past participle. Maybe this is to express the wish that the person
you address has already buggered off!

"Oprotten" is one of those verbs that you have to break up when conjugating.

Examples:
1. "Rot op!": sod off!
2. <infinitive> "Oprotten!" : sod off!
3. <past participle> "Opgerot!" : sod off!

Expression:
"Dat kan me niks rotten!"

<rude:> "I don’t care / I don’t give a damn!" (Literally: "That can rot nothing to me!")

Related word:
A less rude way of saying "oprotten" is "opkrassen" ("to scratch off/away").