"make filthy," c.1280, from O.Fr. defouler "trample down, violate," from de- "down" + fouler "to tread," from L. fullo "person who cleans and thickens cloth by stamping on it." Sense infl. by foul (q.v.); spelling infl. by obsolete native befile, which it replaced and which meant about the same thing.
"narrow passage," 1640s, especially in a military sense, "a narrow passage down which troops can march only in single file," from Fr. défilé, n. use of pp. of défiler "march by files."