In a sexual context, a key holder is the dominant partner in a BDSM dynamic who controls their submissive partner's sexual gratification through the use of a chastity device.
My other half got a new cage for me, which she fitted last night and showed me off to our neighbours. It looked to be the same size but with bars running the length of my penis but this morning she decided it looked bigger than she expected, so she swapped me back into the original cage. It is smaller, which is how she prefers to see me. The neighbours will get a viewing this evening