The term is "spooling", indicating that the data to be sent to an external device has been rapidly placed in a special buffer area. In a system with a computer, the computer is usually able to operate faster than any of the devices attached that data gets sent to or input from. In order to avoid slowing the computer while sending output to, say, a printer, the entire block of data is accumulated so it can be sent to the printer at the printer's rate.
It's a term that is mostly used by chronologically-gifted individuals and is mostly obsolete in the software world. The term "buffer" and "buffering" are more popular. I worked at a disk drive division of a large company in the 80's and you'd also occasionally hear "the disk spooling up" to refer to the disk motor spinning the disk up to speed (no doubt it came from spooling up a jet engine).