"Beside myself with joy"

What does the expression, "Beside myself with joy", mean.

Thanks

It means experiencing extreme joy. "Beside" was formerly (15th through 19th centuries) used in phrases to mean "out of a mental state or condition, as 'beside one's patience, one's gravity, one's wits'" (Oxford Engl. Dict.), and that use survives only in "'beside oneself': out of one's wits, out of one's senses.""