"muddy, foul with extraneous matter, thick, not clear," used of liquids having the lees disturbed or colors, 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.