어 끼우다

swap it in, slot in

끼우다 is fitting one thing snugly into another, and the "-아/어" form in front says what you're doing — mostly 갈아 "replace" or 바꿔 "change". Literally: swapping in headphones, changing the bait, swapping whole letters around. The lines here lean heavily on one joke, 자아 갈아 끼우다 — swapping in a different persona, done in 0.1 seconds — and its variant 성대 갈아 끼우기, changing out your vocal cords. It is the word for a part that clicks into a slot.

Register: casual, often joking