siððan [OE]
sithen, sithenes, synnes, sinnes [MI]
since [ME]
https://www.etymonline.com/word/since#etymonline_v_23549
https://www.merriam-webster.com/dictionary/since#dictionary-entry-3
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED40578
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED40575
https://bosworthtoller.com/27885