Under the cosh
A cosh is a short, weighted stick or truncheon, commonly carried by street ruffians, and dates from 1869, according to the OED. The origin is unknown, although it could be echoic of the sound such an implement might make if used to strike another person. To be under the cosh has acquired a figurative meaning from the mid-20th century and means to be under pressure or duress, presumably from the allusion of being threatened by a cosh. See also Under the pump, under the gun.