an offering of liquor made to a god during the harvest festival, called "black libation"
黒御酒という,神にその年の収穫を供える祭りで供える酒 - EDR日英対訳辞書
It is said that when he was a child, Gennai created a work depicting a 'sacred libation' by tooling a hanging scroll.
幼少の頃には掛け軸に細工をして「神酒」を作成したとされる。 - Wikipedia日英京都関連文書対訳コーパス