"vessel perforated with little holes to allow liquid to run off," mid-14c., coloundour, probably altered (with unetymological -n-) from Medieval Latin colatorium "strainer" from Latin colatus, past participle of colare "to strain," from colum "sieve, strainer, wicker fishing net," which is of uncertain origin.
Cognate with French couloir, Spanish colador, Italian colatojo. The word in English had a wide range of spellings (cullender, coloner, cullyandre, etc.), reflecting uncertainty of the etymology. "The form of the Eng. word appears to be due to some perversion; but its exact history is obscure" [OED]. As a verb, "to pass through a colander," 1874; earlier "riddle with holes" (1862). Related: Colandered.