I suspect that De’vID is right.  I thought that Okrand may have mentioned it in one of his many posts, articles and explanations. but when I searched all of my notes for the phrase I found nothing. 

 

--Voragh

 

From: tlhIngan-Hol On Behalf Of De'vID
Sent: Thursday, November 4, 2021 5:14 AM

On Tue, 2 Nov 2021 at 04:58, Scott D. Randel wrote:

Lately, I have heard people translating {-vaD} as “for the benefit of.” 

 

Lately? According to the mailing list archives, it's been used since (at least) 1996:

https://www.kli.org/tlhIngan-Hol/1996/May/msg00326.html

 

I suspect, as others have already noted, that it probably just comes from a rephrasing of the definition in TKD itself.