selly
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.
Where it comes from
From Middle English selly, selly, sellich, from Old English sellīċ, seldlīċ (“rare, strange, wondrous”), from Proto-West Germanic *seldalīk, from Proto-Germanic *seldalīkaz, equivalent to seld + -ly. Cognate with Scots selly, silly (“approved, good, worthy”), Old Saxon seldlīk (“rare, wonderful”), Gothic 𐍃𐌹𐌻𐌳𐌰𐌻𐌴𐌹𐌺𐍃 (sildaleiks, “wonderful”).
Adapted from the Wiktionary entry selly (CC BY-SA 4.0).