Filth and old age, I’m sure you will agree, are powerful wardens upon chastity.

The line is a sardonic observation that sexual restraint is often enforced less by virtue than by circumstance. “Filth” (squalor, bodily uncleanliness, or social degradation) and “old age” are figured as “wardens” that keep chastity in place by making desire or sexual opportunity less likely. In Chaucerian terms, it fits the tradition of earthy, anti-idealizing comedy that punctures courtly or moral pretensions: chastity can be the byproduct of diminished attractiveness, poverty, or physical decline rather than a freely chosen moral discipline. The humor depends on blunt realism about the body and on the cynical implication that many people would not be chaste if they had youth, cleanliness, and opportunity.