An uppercut is a punch that travels upwards — the fist comes from low down and drives up into the other person's chin or body. It is a boxing word first of all, and boxing has names for each kind of punch: a jab goes straight out, a hook swings in from the side, an uppercut comes up from below. The reason it has its own name is that it works differently: because it lands under the chin, it snaps the head back and is one of the punches most likely to knock someone out. It is mostly a noun — 'he threw a left uppercut', 'she caught him with an uppercut' — and it can be a verb too, though that is much less common. Outside the ring, you will hear it in fighting video games, where the uppercut is usually a big special move, and in newspaper writing where a sudden sharp attack in politics or business is called an uppercut. That figurative use is journalist-style, not everyday speech. Note that the word is about the direction the punch travels, not about which hand you use, so 'left uppercut' and 'right uppercut' are both normal.