noun
elder sister; big sister
Humble term used to refer to one's own elder sister when speaking to someone outside the family. Also used in yakuza or gang contexts to refer to a senior female member, or affectionately to an older woman one respects.
姉御がいつもお世話になっております。
My elder sister is always in your care.
組の姉御はとても面倒見がいい。
The gang's big sister is very caring.
Compound of 姉 (ane, 'elder sister') and 御 (go, honorific suffix). The exact historical development is uncertain, but it likely arose as a respectful or humble way to refer to an elder sister within certain social groups.