ADJ.
sick, sore
SELLY + NOUN
sight
PREP.
in
adj
Rare; wonderful; admirable
adv
Wonderfully, wondrously.
His brother was […] selly sick and sore unsound.
noun
A marvel; wonder; something wonderful or rare
The line is a masterstroke of noncommitment, for the event is a "selly" in the sight of some unidentified readers.
i Register
In some senses, selly is marked as UK. Watch for register when choosing this word.