a child that one's husband or wife had by a former marriage
someone or something regarded as unpopular, neglected, etc.
Origin:
ME < OE steopcild: see step- & child
See stepchild in American Heritage Dictionary 4
noun
A child of one's spouse by a previous union.
Something that does not receive appropriate care, respect, or attention: “Demography has a reputation for being the stepchild of . . . economics”(Louis Pol).