A Macintosh user who is still using Mac OS X "Panther", in spite of there now being two newer versions (Tiger, Leopard)

To pleasure onses self to old pictures of celebrities/porn stars who are currently past there prime.

Coolest gang out there. Have the best butts and they all throw it back so hard. All members of throw it back gang are fit and sexy. The throw it back gang can get any boy they want.

When you sexually please a girl from behind

When felling a tree, a Tijuana back cut is when you do not cut a notch, just a straight back-cut.

Term only applies when felling entire tree

To back can be just defined as to support.
But it can also be slang for pulling out some kind of weaponery like a knife,

this is mostly used by people in Gangs or people involved in the streets.