From se. Related to Finnish silloin. This etymology is incomplete. You can help Wiktionary by elaborating on the origins of this term.
siloi
- then, at that time
- then, in that case
- Zajceva, N. G., Mullonen, M. I. (2007) “тогда”, in Uz’ venä-vepsläine vajehnik / Novyj russko-vepsskij slovarʹ [New Russian–Veps Dictionary][1], Petrozavodsk: Periodika