Literally, bending a circuit in some way to change what it does. This refers to the adventurous and tech-savvy arming themselves with electrical engineering knowledge, the proper tools and parts, and opening up analog musical equipment to modify and rewire it in the hopes of creating interesting and unique sounds.
Circuit bending was particularly popular in the early days of synthesis when many synths were constructed from kits, but it’s still popular today among the adventurous, skilled, and tech savvy. The recent Korg Monotron analog synthesizer has become a playground for circuit benders, but there are lots of online communities devoted to circuit bending both vintage and modern modular synthesizers and guitar pedals, noise-making toys, and other sound-generating electronic equipment.