Hi,
Below is another example of the "past imperative", as it
may be called.
As I began writing, I wasn't sure where the quote was from.
But right now I managed to locate it in king Olaf's saga:

« Þá hljóp að Áslákur Fitjaskalli og hjó með öxi
í höfuð Erlingi svo að stóð í heila niðri.
Var það þegar banasár. Lét Erlingur þar líf sitt.

Þá mælti Ólafur konungur við Áslák: "Högg þú
allra manna armastur. Nú hjóstu Noreg úr hendi mér."

Áslákur segir: "Illa er þá konungur ef þér er
mein að þessu höggi. Eg þóttist nú Noreg í hönd
þér höggva. En ef eg hefi þér mein gert konungur
og kanntu mér óþökk fyrir þetta verk þá mun mér
kostlaust vera, því að hafa mun eg svo margra
manna óþökk og fjandskap fyrir þetta verk að eg
mundi heldur þurfa að hafa yðart traust og vináttu."
»

(it is always good to have the context, when trying
to grasp meaning.)

The quote concerns the killing of the viking chief
Erling Skjalgsson by one of king Olaf's men, whose
name was "Aslak Fitjaskalle". Olaf wanted to gain control
over all of Norway, but Erling stood in his way as the
mightiest man in Norway. At one point king Olaf had him
cornered, though, somewhere on the upper deck of his ship.
It was, however, not in the king's interest to have him
killed. What surprise then, when one of the king's men -
the man called Aslak - suddenly jumps forward and plants
his axe into Erling's skull, causing Erling to die almost
instantaneously ; upon which the king immediately exclaims:
" Now look what you've done, you idiot! Who told you to
attack him? We were just about to start negotiations
and he had trusted us that it was okay to put his weapons
down! And then you suddenly jump forth without being
asked. Look what you've done: You have actually caused
me to lose all of Norway instead of gaining it! "

(this is what the dialogue would have sounded like in our day)
But of course the vikings spoke much more briefly,
so that even in the heat of battle, every word that
issued from their mouths was poetry and gold !
Hence the King Olaf *actually* said :

" Högg þú allra manna armastur. Nú hjóstu Noreg úr hendi mér. "
(Hew thou poorest of all men! Now you hewed Norway out of my hand.)

So you see that he is applyinging the IMPERATIVE of höggva
(=to hew/chop) to a situation or an act that has just been
completed, which is then an example of the so called "past
imperative", that was already discussed in another post.

Below I have gathered a few examples of how different
authors of ON grammar books have translated the phrase

« Högg þú allra manna armastur. »

O.E. Haugen "Hogg du, uslast av alle!" (Hew! you wretched creature.)
"Berre hogg du, din usling." (Keep on slashing, you wretched
creature!)
A. Heusler "Hau als unseligster der Menschen!" (Strike as the most
wretched among men!)
"Sei verwünscht für deinen Hieb!" (Be accursed for your stroke/blow!)
R. Iversen "Hogg du fattigste av menn!" (= literal)
(Hew thou poorest of men!)
"Forbannet være ditt hogg!" (= translation of meaning)
(Accursed be thine hew/slash!)

(The imperative applies to a just completed act)
It is just as if the king says "Keep on slashing, you wretch!"
In this case the damage has already been done, and
the imperative serves as a request to confirm the
content of the assertion. (OEH p. 278)
Another name is "Imperative of Wish". (H. 424)

For grammatical details, note that the two words
'allra manna' which is merely a masculine genitive plural
(= 'of all men') may be dropped without changing the
meaning of the phrase. Hence we are left with the
simplified sentence:

« Högg þú armastr. »
(Hew you poorest)

And note the structure of imperative 2nd sg. + pronoun þú.
The personal pronoun always immediately follows the imperative.
Then at the end of the sentence there is a adjective,
here 'armastr', which happens to be a superlative, but a
postive will do just as well, e.g. « Högg þú armr! »
(Hew! you poor one) will do just as well.
But note that the adjective is declined in the masculine
singular Nominative form. That is, the adjective has to
conform in gender, number and case to the subject þú.
I therefore believe that the sentence may be formally
emended by adding the words 'ok ver þú'. If that is correct,
one may then 'improve' upon the sentence by writing it as

« Högg þú, ok ver þú allra manna armastr. »
(Hew thou, and be thou the poorest of all men!)

By 'improving' I merely mean that we make its structure
more transparent to us who are still learning the language.
Thus, what the king is saying, may be understood as
"Keep on hewing! and be thou [thereby] the most wretched of
all men!"

It is as if by encouraging the villain to keep up his
"fait accompli", the speaker may thereby attach an ill
wish to his statement, which thereby acquires the form
of a cursing.

With best greetings
Xigung.