堕 is a formal verb that covers both physical falling/dropping and moral or social decline. Its most common modern senses are 'to fall (physically)'—as in objects or people falling from a height—and 'to degenerate' or 'to become corrupted'—as in moral depravity (堕落). You also see it in 堕胎 (abortion). The word is more literary than 掉 or 落 when describing physical falling.