> Þessi atburður varð með svo skjótri svipan (acc sg of svipr = a
sweeping movement, Z1) að engi kom höggi á hann.
> This attack happened with such a speedy swipe that no blow came on
him. He ran at-once away down with-violent-speed.
'svipan' is dative singular of the feminine noun 'svipan', meaning the
same thing here. The adjective 'skjótri' has the feminine dative
singular inflection -ri, agreeing with 'svipan'. Compare 'með svá
skjótri svipan' with Zoega's example 'með svá harðri svipan' "so quickly".
The masculine noun 'svipr' becomes 'svip' in the accusative (or
svipinn with the suffixed article). I guess you were thinking of the
masculine accusative adjectival ending -an?