sincesince

Since comes from Middle English syns, a contraction of sithens, from sithen ‘after, since’, from Old English sīþþan, from the phrase sīþ þǣm ‘after that (time)’.

1. Old Englishc. 450–1150sīþ þǣm¹‘after/since that (time)’
2. Old Englishc. 450–1150sīþþan¹‘afterwards, since’
3. Middle Englishc. 1150–1500sithen¹‘after, since’
4. Middle Englishc. 1150–1500sithens¹
5. Middle Englishc. 1150–1500syns¹
6. Englishearliest quotation c. 1596–1599since¹

1. ¹ Wiktionary, English since
2. Earliest quotation in Wiktionary’s entry: c. 1596–1599, William Shakespeare, The Second Part of Henry the Fourth, […]. Epilogue.: “O ſir Iohn, doe you remember ſince wee lay…”.