Merriam-Webster Collegiate Dictionary - happen

 intransitive verb  (~ed; ~ing)  Etymology: Middle English, from hap  Date: 14th century  1. to occur by chance — often used with it it so ~s I'm going your way  2. to come into being or occur as an event, process, or result mistakes will ~  3. to do, encounter, or attain something by or as if by chance I ~ to know the answer  4.  a. to meet or discover something by chance ~ed upon a system that worked — Richard Corbin  b. to come or go casually ; make a chance appearance he might ~ by at any time  5. to come especially by way of injury or harm I promise nothing will ~ to you