If you spend time in kernel mode, you're accustomed to seeing functions with two-letter (or occasionally, three-letter) prefixes that indicate which component they belong to.

PrefixComponentExample
Ex Executive ExAllocatePool
Hal Hardware abstraction layer HalGetBusData
Io I/O manager IoAllocateIrp
Ke Kernel KeBugCheck
Ks Kernel streaming KsAcquireControl
Mm Memory manager MmGetPhysicalAddress
Ob Object manager ObReferenceObjectByHandle
Po Power management PoSetSystemState
Se Security SeAccessCheck
Tdi Transport driver interface TdiProviderReady
Zw ???? ZwCancelTimer

What does the "Zw" mean?

Answer: Nothing.

The people who chose the letters wanted to pick something that was unlikely to collide with anything. Perhaps they had a prior bad experience with having chosen a prefix, only to find that somebody ahead of them claimed it already?