It's all horrendously confusing. Originally a 'sigma-tau ligature' (I'm assuming this means it was a 'st' sound; hence the name), then borrowed as a name for the cursive form of 'digamma' (a 'w' sound) because they looked kinda the same. Then borrowed again as a name for the final sigma in modern Greek.
Modern unicode has a 'final sigma' which it notes is also known as 'stigma', an actual 'stigma' and (rather bizarrely) a 'lowercase stigma'.
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("koppa" = "copper"... oh never mind. Rule 2 of humour: if it has to be explained it was crap to begin with.)
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Hyuk hyuk!
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Modern unicode has a 'final sigma' which it notes is also known as 'stigma', an actual 'stigma' and (rather bizarrely) a 'lowercase stigma'.
Final sigma: ς
Stigma: Ϛ
Lowercase stigma: ϛ