Meaning

noun a narrow pass (especially one between mountains)

verb place under suspicion or cast doubt upon

verb make dirty or spotty, as by exposure to air; also used metaphorically

originFrom Late Middle English defilen (“to make dirty, befoul; rape; abuse; destroy; injure; oppress”) [and other forms], a variant of defoulen (“to make dirty, defile, pollute; have sexual intercourse with; rape; etc.”) (compare also defoilen). Defoulen is a blend of Middle English foulen (“to make dirty, soil, pollute”) (from the adjective foul (“dirty, rotten, stinking, corrupt, sinful, guilty”)…

Etymology from Wiktionary, CC BY-SA 3.0