Dulls

Neither a borrower nor a lender be; / For loan oft loses both itself and friend, / And borrowing dulls the edge of husbandry.

Neither a borrower nor a lender be: For loan oft loses both itself and friend; And borrowing dulls the edge of husbandry.

Alcohol dulls the mind.

Idleness can dull the sharpness of one's abilities, just as rust dulls the edge of a razor, leading to regret and loss of potential.