1580s, "not seen or heard clearly," from Latin indistinctus "not distinguishable, confused, obscure," from in- "not, opposite of" (see in- (1)) + distinctus (see distinct). From c. 1600 as "not clearly defined or distinguished." Indistinctly is attested from c. 1400 in an obsolete sense "equally, alike, indiscriminately." Related: Indistinctness.
