When a mom says something nice about her son and inadvertantly burns his friend in a way so clever that you wonder if it was actually intentional.
friend: "chris is a bad speller"
mom: "it's because he speaks french"
friend: "oh man, that was a total mom burn, I'd trade my mad spelling skillz to speak french"
a halarious burn, usualy made by mothers.
girl: i'm so smart, i can spell better than my friend
friends mom: yeah, well, he can speak french
girl: ouch, that was a total mom burn. i've been taking french lessons twice a week for two years and still can't speak french