Happy primarily refers to something that comes "by good hap," a chance that brings prosperity, benefit, or success.
And grasps the skirts of happy chance.
Tennyson In Memoriam lxiii, st. 2.
In this sense happy is closely allied to fortunate and lucky. (See FORTUNATE.) Happy has, however, so far diverged from this original sense as to apply to advantages where chance is not recognized, or is even excluded by direct reference to the divine will, when it becomes almost equivalent to blessed.
Behold, happy is the man whom God correcteth.
Job v, 17.
Happy is also applied to the ready dexterity or skill by which forable results (usually in minor matters) are secured, when it becomes a synonym for dexterous, felicitous, and the associated words; as, he has a happy wit; happy at retort (compare CLEVER). In its most frequent present use, happy is applied to the state of one enjoying happiness, or to that by which happiness is expressed; as, a happy heart; a happy face; happy laughter; happy tears (compare synonyms for HAPPINESS). Cheerful applies to the possession or expression of a moderate and tranquil happiness. A cheery word spontaneously gives cheer to others; a cheering word is more distinctly planned to cheer and encourage. Gay applies to an effusive and superficial happiness (often not really worthy of that name) perhaps resulting largely from abundant animal spirits: we speak of gay revelers or a gay horse. A buoyant spirit is, as it were, borne up by joy and hope. A sunny disposition has a constant tranquil brightness that irradiates all who come within its influence.
Compare synonyms for GRIEF.