An expression used by the groom-to-be's side of the family during a wedding planning to describe the bride's control-freak of a mother. If the bride is "Bridezilla," then her mother is "Mother-In-Lawthra" (a reference to Godzilla and Mothra of the old Japanese monster movies).
Hey, dude, you're lucky your fianceé isn't turning into a bridezilla.
True, but her mother's already a Mother-In-Lawthra.