Filer un mauvais coton

Literally: To spin a bad cotton

Example: “Depuis qu'il a arrêté de sortir, mon grand-père file un mauvais coton.” — Since he stopped going out, my grandfather has been going downhill.

English equivalent: To go downhill

Origin: Nothing to do with tailing a suspect: the filature here is the mill where raw cotton is spun into thread. In the late 17th century the phrase was jeter un vilain coton (to throw off an ugly cotton), from the way worn fabric sheds little balls of fluff as it goes — the visible sign that a cloth is on its way to falling apart. Across the 19th century vilain gave way to mauvais, and as spinning mills multiplied, jeter gave way to filer (to spin). The meaning had by then already widened from ruining oneself to failing in health, and it now covers anything visibly heading downhill.