Your forefinger is the finger next to your thumb — the one you point with. People also call it the index finger. The fore- piece means “at the front”: on an open hand, this is the finger that stands right beside the thumb, toward the front. You use it to press a button, turn a page, or test whether soup is hot. It is a plain body-part word, not slang. Though actually, in everyday talk many people say index finger, or they just hold up that finger and say this one. Forefinger sounds a little more like a book or an older speaker, but everyone still understands it.