Hello friends. Haraldr harðráði in old Icelandic means Harald Hard-buttocks. Yes, you've got that right. harðráði is a perjorative in some texts for a mean ruler and just as today it is in reference to a man's hind quarters.
I should have said you can find the perjorative use of harðráði in the Sagas. Of course, this is a rare double entendre ... the "radt" as parliament is featured in a number of northern European languages...
I actually took Icelandic for a semester, and my Icelandic dictionary says: "ráða (ræð; réð; ráðinn) v.t. advise; recommend; ráða e-m áð gera e-ð, advise s.o. to do s.t.; with dat. rule, govern." Nothing about bottoms anywhere.
It seems an obvious cognate with the Anglo-Saxon ræd and Middle English rede, all meaning the same thing - counsel or advice. I would think the Icelandic for the hinder parts would be a cognate of the A/S "buttuc" which means what it sounds like.