Colder Than My Mother-in-Law’s Heart

Origin

This expression builds on a long Southern and country tradition of colorful exaggerations - "colder than a well digger’s behind," "hotter than blue blazes," "slicker than owl snot," and the like.

The "mother-in-law’s heart" variation emerged in **20th-century rural humor**, likely from jokes about difficult in-laws. It caught on because it blends warmth, wit, and exaggeration - hallmarks of Southern storytelling.

Verdict: Southernish. Modern, humorous, and rooted in the country style of one-liners and porch talk.