BT Sot 34a(39) :דטעונה דמדלי איניש לנפשיה לכתפיה תילתא דטעונה הוי‏ : a load that a person can raise to his shoulder by himself is a third of the load (which he can carry if placed on it by another)
See the entire entry for ṭˁūn, ṭˁūnā n.m. load