Arabic

fšḫ - II. avoir les membres du corps lâches, sans vigeur (to have loose body members, without vigor) (BK 2 592)

Related as a variant root with š vs. s and a meaning shift?

*psḥ/ḫ - to be lame; have a dislocated limb (SED I No. 54ᵥ; HALOT 947)