My favorite Latin quote, mostly because it's so appropriate these days in many, many places.
Yes, Koala, quote is right. According to my sources¹, it's from Juvenal's misogynous satire (6,365, O 31-2). Juvenal pointed out that it's useless to hire a guard to guard a wife of questionable loyalty, because guards can be bribed (And, as pointed out by evilrooster, be the goats guarding the cabbage patch - good point).
These days, of course, guards are not always hired to guard the women only, but the guarding ability and trustworthiness of guards can always be questioned.
¹ Arto Kivimäki: Carpe Diem! Hauskaa ja Hyödyllistä Latinaa, Karisto, ISBN 951-23-3661-8