SHMILY stands for "See How Much I Love You". My husband has one identical to it on the opposite ankle. The celtic knot is a triquetra, also the same symbol of "the power of three" from "Charmed".
I like it, in general. However...does your husband like Broadway musicals? Just checking.
He needs to let everyone know that he wanted to say 'smiley' but was too drunk after a night of gambling & whoring. More manly.