v. To surreptitiously perform a task in the background while one's attention is supposed to be on another task.
2009
Backgrounding: doing one primary thing while some other things are going on in the background. Listening to music and reading a book. Listening to a podcast and perusing the news. These generally use different parts of the brain and one can argue whether the primary task suffers or not.
1999
backgrounding verb The practice of not giving someone your full attention while multitasking.
1999 (earliest)
Ever rung a colleague or client and found that the only response your questions are soliciting is the occasional "Hmmm"? You've just been "backgrounded"! To background someone is to deny them your full attention while multi-tasking.