What is the X in X-mas?
1570s, from Late Latin nephritis, from Greek nephritis "disease of the kidneys," from nephros "kidney" (see nephron) + -itis "inflammation." Related: Nephritic.
nephritis ne·phri·tis (nə-frī'tĭs)n. pl. ne·phri·tis·es or ne·phrit·i·des (-frĭt'ĭ-dēz') Acute or chronic inflammation of the kidneys.