1520s, in reference to cargo, from un- (2) + load (v.). Figurative sense (in reference to feelings, etc.) is recorded from 1590s. Used in reference to sales of stocks by 1870, hence U.S. colloquial sense "dispose of property the holding of which is risky" (1881). Related: Unloaded; unloading.
[unload etymology, unload origin, 英语词源]