custos

Custos, custodis is third declension and common gender: masculine or feminine according to who is doing the guarding. The stem is custod-, and the declension trap is the genitive plural, which is custodum, not custodium. Despite the look of the nominative it is a consonant stem, not an i-stem.

The verb beside it is custodio, custodire, custodivi, custoditum, "guard, keep", fourth conjugation, and the abstract is custodia, "custody, guard duty, prison". Juvenal's "quis custodiet ipsos custodes" is where most English readers meet the word, and its original subject was not political oversight but the practical problem of hiring men to keep watch on your wife.