KHASR خصر
Lit. “The middle of waist.” An act forbidden in prayer as related by Abu Hurairah, who said: “The Prophet forbade Khasr in prayer.” (Mishkat, book iv. ch. xx.) It is generally held to be the act of holding the waist with the hands to relieve the sensation of fatigue experienced in the position of standing. Some divines believe it to be a prohibition to lean on a mikhsarah, or staff, in prayer, whilst others give to it the sense of cutting short the verbal forms of prayer, or remaining too short a time in the prescribed attitude. (Shaikh ‘Abdu ‘l-Haqq.)

Based on Hughes, Dictionary of Islam