Selfishness, or rational self-interest, is the ethics of the Objectivist philosophy. To be "selfish," according to the Oxford Modern English Dictionary, means to be "concerned chiefly with one's own interest." Notice that the definition contains no ethical evaluation, either positive or negative.
Ayn Rand chose the word "selfish" because it most closely denotes the concept of living for one's own sake rather than living primarily for the sake of others. No other word quite captures that idea.
The fact that most people think that being selfish means harming one's fellow man, that pursuing one's own self-interest equates to behaving brutally or irrationally, is, as Ms. Rand noted, a "psychological confession" on their part. In fact it is against one's own long-term self-interest to behave irrationally or trample others. Such actions are the exact opposite of selfish--they're self-destructive.