1758, of fluids, from in- (2) "in" + filtrate. Related: Infiltrated; infiltrating. Military sense of "penetrate enemy lines" attested from 1934.
infiltrate in·fil·trate (ĭn-fĭl'trāt', ĭn'fĭl-)v. in·fil·trat·ed, in·fil·trat·ing, in·fil·trates
To cause a liquid to permeate a substance by passing through its interstices or pores.
To permeate a porous substance with a liquid or gas.