What does "mult" stand for in the title "Prof. Dr. Dr. h.c. mult"?

I know h.c. is Honoris Causa. But I cannot work out what "mult" is. I need well-grounded answers so please give source reference(s).

What does "mult" stand for in the title "Prof. Dr. Dr. h.c. mult"?
I'm afraid I can't provide you with a citation, but I can at least tell you that the "mult" designation indicates that the person in question is the recipient of multiple honorary degrees, lame as that sounds. It's a European convention--once you earn a first "h.c." designation, the "mult" designation immediately attaches as soon as you earn a second. I scanned the pages of a few profs to confirm, and that's the best proof I can offer (I did some Google hunting and found as little as you did): you'll find all the folks with the mult designation have multiple honorary degrees to their credit.