真身 has a strong religious origin: a Buddha or bodhisattva temporarily takes a
化身 (huàshēn, manifestation form) to walk among mortals; revealing
真身 is the dramatic moment they show their cosmic, original body. Famous example — Famen Temple's
佛指真身舍利 (Buddha-finger relic). Modern internet usage borrows the metaphor: revealing your
真身 = unmasking the real person behind an online avatar.