1. A person who uses his or her computing skills to compromise or destroy corporate or private computer files. Their techniques can vary from using advanced programming skills and social engineering to utilizing "out-of-the-box" scripts developed by others. The latter is often referred to as a script kiddie. The term seems to have developed in emulation of hacker with an additional connotation of forcible as opposed to clever. See The Jargon File maintained on the 'Net by Eric S Raymond (at www.tuxedo.com), and published on paper by MIT Press, for a view of the history of hacker and hacking.
2. A dry, thin and crisp savoury biscuit - US origin.
3. A disparaging term for a white person (usually one living in the American south, and usually one who is poor, and most likely with a socially 'conservative' viewpoint) - US origin.
4. An American alternative rock band, fronted by former Camper Van Beethoven signer Dave Lowery. They had a few minor hits in the early 1990's, including Teen Angst (What the World Needs Now) and Low. Though still a going concern, they have essentially been a cult act since about 1995.
The general public will be much more familiar with senses 2 and 3 than with 1 and 4.