Nipto is the verb for washing a part: the hands before bread, the face after fasting (Matthew 6:17), the feet after the road. When the Septuagint reached the bronze laver, where Aharon and his sons wash their hands and feet before they serve, it chose nipto (Exodus 30:19). The priests were bathed whole once, at ordination; at the laver, for the rest of their service, they rinsed the parts that met the world.

English folds it into the same "wash" that translates louo, and in John 13 the fold costs the reader the architecture of the scene. The KJV's "wash" covers both verbs, so the basin and towel look like a lesson in humility and nothing more.

It is that, and it is more. The Master kneeling with the basin is doing laver work, nipto work, the daily rinse of feet that belong to men already bathed. "He that is bathed needs only to have his feet washed, and is wholly clean" (John 13:10). The once-for-all and the daily are two different verbs in His sentence, and neither can do the other's job. The one who confuses them either re-immerses forever or walks with dirty feet.