Context, Ragnars saga loðbrókar, chapter 1: a covertous old peasant
woman is urging her dubious but easily swayed husband to sneak up on
their supposedly rich guest while he's sleeping, and do him some
mischief with an axe.


þá mælti kerling til karls, at hann skyldi láta verða tilræði sem
bezt, - "ok skunda brott með hlaupi, því at ekki máttu standast lát
hans ok óp, ef hann fær þik höndum tekit."

Then the old woman said to the man/peasant that he should MAKE HIS
ATTACK BE BEST, "and hurry away at a run, because you won't be able
to stand/withstand his roar and his shout, if he gets you with-hands
taken."

Then the old woman said to her husband that he should make the most
of his attack, "and run away fast, because you won't be able to
stand his roar and his shout, if he gets a hold of you."

http://www.snerpa.is/net/forn/ragnar.htm


In other words: make his first blow count, because if he wakes up
without being incapacitated, you won´t get a second chance. 'lát
hand ok óp' refers to Heimir´s supernatural death-shout. Though
wrong about most things, she's right about this: it comes to about 7
on the Richter Scale:

http://www.volcanolive.com/magnitude2.html
http://hraun.vedur.is/ja/index.html?