plenus

Plenus, plena, plenum is a regular first and second declension adjective. What fills it usually goes into the genitive - plenus hominum, plena aquae - and the ablative is the common alternative, especially with things you measure: plenus vino.

Both constructions are correct, which is worth saying because the single most quoted instance of the word, gratia plena in the Ave Maria, is ablative and looks wrong to anyone taught the genitive rule as absolute. It is not wrong; it is the other option. The verb family is impleo, compleo, repleo, and the noun is plenitudo. English took plenty, plenary, plenitude and, at some distance, "replete". Pleno gradu, "at full stride", is the army's phrase for quick march.