Green’s Dictionary of Slang

1. (US) to be in one’s element.

1872		Schele De Vere Americanisms 642: If you talk about hunting for meat, I am there; if you want me to follow a trail, I am there, and, by Jingo, if you want me to snuff you out as you are, I am there too.
1902		Ade Forty Modern Fables 180: When it comes to being there with the Gray Matter you have got all of us Old People left at the Post.

2. (US) to make a definite plan; usu. in the form of a statement, I’m there, I’m definitely doing that.

1870		B. Harte Luck of Roaring Camp n.p.: Want me to liquor, stranger? I am thar.
1874		‘A.P.’ [Arthur Pember] Mysteries and Miseries 348: [I] said that I had fifteen cents left and was willing to stand a quart of ale [...] ‘Beer! I’m there,’ exclaimed dirty ruffian number two.
1983		Eble Campus Sl. Nov.
1989		P. Munro Sl. U. 33: Michael Jackson is giving a concert at the Forum. I’m there!

3. (US campus) to understand.

1993

Eble Campus Sl. Oct. 1: be there – understand: X: ‘Did you understand my joke?’ Y: ‘We’re there, dude.’.

4. of a man, to seduce, to have sexual intercourse.

2005

P. Howard Curious Incident of the Dog in the Nightdress 29: Have you actually been there, or even tried to be?