Archaic interjection expressing agreement or confirmation. Equivalent to modern それそれ (sense 2).
See also: それそれ
"Yeah, that's right," the old man nodded.
The exact derivation is uncertain; it may be a reduplicated form related to それ (that), used as an interjection. The kanji spellings 其々 and 其其 are historical and not used in modern Japanese.