knob 1 a round handle on a door or a drawer. 2 a round switch on a machine such as a radio that you use to turn it on and off, etc. 3 round shape like a ball on the surface or end of something. 4 🇬🇧 a small piece of something such as butter. with knobs on (idiom, slang, 🇬🇧) Used to say that something is a more complicated version of what you mention: 'It isn't art—it's just a horror movie with knobs on!'