What does Boxing Day have to do with boxing?
1756, "an unfolding;" see develop + -ment. Of property, with the sense "bringing out the latent possibilities," from 1885 (Pickering's glossary of Americanisms, 1816, has betterments "The improvements made on new lands, by cultivation, and the erection of buildings, &c."). Meaning "state of economic advancement" is from 1902. Meaning "advancement through progressive stages" is 1836.
development de·vel·op·ment (dĭ-věl'əp-mənt)
The act of developing.
The state of being developed.
A significant event, occurrence, or change.
The natural progression from a previous, simpler, or embryonic stage to a later, more complex, or adult stage.