Back in the day, it wasn't uncommon for the key to directly switch the transmitter. Transistors didn't exist, and why would you use a relay when you already have a perfectly good switch under your fingers?
Also back in the day, it wasn't uncommon to transmit with a whole ton of power. Building a bigger transmitter is technologically less challenging than building a more sensitive receiver.
So, the full power of a very large transmitter going through your key -- you might imagine what happens. Between mechanical wear from sending all those telegrams, and arcing from using a key to switch many kilowatts of transmitter power (probably with no attempt to suppress the inductive arcing), the key contacts inevitably get mutilated. Imagine what a resistive, scratchy, intermittent switch contact does to your signal. "Mutilated" seems like as apt an adjective than any.
These days the switching is done with relays or transistors, so if you encounter "QSD", the most likely explanation is that you have traveled back in time to the early 20th century.