In Russian, подлец (podlets), "scoundrel" (our translation), is the base, vile man, and it is the word Mitya applies to himself since he spent Katerina Ivanovna's money. He separates it from вор (vor), "thief", and for the thief creates the word архиподлец (arkhipodlets), arch-scoundrel: anyone can be a scoundrel, only the arch-scoundrel is a thief. At the trial he says that Dmitri Karamazov is a scoundrel, but not a thief.
Where It Appears
Quotes
I am a pickpocket, I am a scoundrel, and I do not want to begin a new life as a scoundrel.
Book VIII: Mitya, Mitya, on the three thousand rubles he owes Katerina Ivanovna
I love Russia, Alexei, I love the Russian God, even though I myself am a scoundrel!
Epilogue: Ilyusha's funeral, Mitya to Alyosha, in the hospital