Green’s Dictionary of Slang

(US) a see-through or diaphanous dress.

1897		Pittsburgh Post (PA) 4 Apr. 22/4: [headline] Dresses of the X-Ray Variety [...] Soft, Diaphonous Material.
1928		N.I. White Amer. Negro Folk-Songs 331: [reported from Durham, N.C., 1919] Mary wore an X-ray dress, / And she left off all the rest.