late 14c., "one who handles" anything, agent noun from handle (v.). Specific sense of "one engaged in trade" is from 1690s; that of "prizefighter's assistant" (1916) was earlier used in reference to dogfights and cockfights (1825).
1. A good handler can lessen faults.
2. The sword - handler took the cape, looking up at Brett.
3. The exception handler chosen is said to catch the exception.
4. Designer verbs associate a menu item with an event handler.
5. In some cases you can dissociate a handler from an event.