1620s, from Latin turbidus "muddy, full of confusion," from turbare "to confuse, bewilder," from turba "turmoil, crowd," which is of uncertain origin. De Vaan writes:
Turba seems most similar to [Greek syrbe, Attic tyrbe] 'noise, commotion', ... which are probably loanwords. In that case, Latin would have borrowed the word from a Greek dialect, or both Greek and Latin borrowed it from a third source. In view of the quite well-developed word family already in Plautus, which suggests that turba had been in the language for some time, the latter option seems preferable.
Related: Turbidly; turbidness.