In the West of Scotland, the term 'buckled', sometimes referred to as 'backlt' is used to express the action of knees buckling due to laughing to an excessive extent. It is rare that someone will actually 'buckle', yet it is still commonly used.
buckled is when someone either looks like, or has actually been punched in the face with a great-big "tombstone" belt buckle--the kind that are typically worn by those on the Professional Bull Riding circuit.