noun
elder sister; big sister
Familiar and honorific term for one's own elder sister, used mainly by younger siblings. Not used for addressing someone else's sister directly.
姉貴が大学に合格した。
My big sister got into university.
姉貴、ちょっと手伝ってくれる？
Hey sis, can you help me out a bit?
noun
older female friend; big sis (figurative)
Used for a slightly older female friend or acquaintance, often in casual or slangy speech. Can carry a rough or gang-like tone when written as 姐貴.
あの人は俺のアネキみたいな存在だ。
She's like an older sister figure to me.
姐貴、今日もよろしく！
Hey big sis, looking forward to today!
Compound of 姉 (ane, 'elder sister') + 貴 (ki, honorific suffix). The kanji 貴 is ateji, used phonetically for its honorific connotation.