1. To cut something off, often with clumsy strokes or stabs. A noun or pronoun can be used between "hack" and "off." That's it—I'm going out and hacking off the part of the bush that's blocking the driveway!

2. To irritate or annoy someone. Primarily heard in the UK. A noun or pronoun can be used between "hack" and "off." He keeps undermining me to the boss, and it's really hacking me off.