"POK" is the infamous sound made by a tennis ball coming in contact with a perfectly placed swing of a tennis raquet.
POK is also the term given to ThumbDrives or USB Drives (USB Mass storage devices).
This term is derived from the brand name of Hewlett-Packard USB Drives called the "PockiDrive".
Shortened, edited and combined with the personified "POK" sound, the term "POK" took over the old USB(Thumb)Drive.
Dude, pass me your POK, I need to save some work on it.