Source : Webster's Revised Unabridged Dictionary (1913)

Happen \Hap"pen\, v. i. [imp. & p. p. {Happened}; p. pr. & vb.
   n. {Happening}.] [OE. happenen, hapnen. See {Hap} to happen.]
   1. To come by chance; to come without previous expectation;
      to fall out.

            There shall no evil happen to the just. --Prov. xii.
                                                  21.

   2. To take place; to occur.

            All these things which had happened.  --Luke xxiv.
                                                  14.

   {To happen on}, to meet with; to fall or light upon. ``I have
      happened on some other accounts.'' --Graunt.

   {To happen in}, to make a casual call. [Colloq.]

Source : WordNet®

happen
     v 1: come to pass; "What is happening?"; "The meeting took place
          off without an incidence"; "Nothing occurred that seemed
          important" [syn: {hap}, {go on}, {pass off}, {occur}, {pass},
           {fall out}, {come about}, {take place}]
     2: happen, occur, or be the case in the course of events or by
        chance; "It happens that today is my birthday"; "These
        things befell" (Santayana) [syn: {befall}, {bechance}]
     3: chance to be or do something, without intention or
        causation; "I happen to have just what you need!"
     4: come into being; become reality; "Her dream really
        materialized" [syn: {materialize}, {materialise}] [ant: {dematerialize},
         {dematerialize}]
     5: come upon, as if by accident; meet with; "We find this idea
        in Plato"; "I happened upon the most wonderful bakery not
        very far from here"; "She chanced upon an interesting book
        in the bookstore the other day" [syn: {find}, {chance}, {bump},
         {encounter}]