A set phrase used when summarizing or omitting details, similar to 'such and such' or 'so and so' in English. Often used in storytelling or explanations.
He said he was late for such and such reasons.
For various reasons, I'm leaving early today.
From 斯く斯く, “thus and thus,” and 然々, “so and so,” a reduplicative formula used in place of omitted particulars.