1640s, "not clearly perceived;" 1650s, "incapable of being told apart," from in- (1) "not, opposite of" + distinguishable. Shakespeare's use of it (c. 1600) seems to mean "of indeterminate shape." Related: Indistinguishably.
they wore indistinguishable hats
the two specimens are actually different from each other but the differences are almost indistinguishable
the twins were indistinguishable
a colorless person quite indistinguishable from the colorless mass of humanity