The Motorola MC68000 CPU has 8 general purpose, and 8 address registers. The last address register can be referred to as A7 according to the standard naming convention or SP, since it doubles as the stack pointer.
The 68000 instruction set is often used in educational settings, because it is in wide use, and considered much clearer than the also widely used Intel x86 instruction set. Due, perhaps, to the undersexed nature of some undergraduate computer science students, this lead to an interesting bit of slang developed in my computer architecture class at UNC Chapel Hill. Since the word stacked can refer especially buxom women, A7 was adopted jokingly as local slang for a hot girl, as in: "Check out that girl over there - A7."
This silliness, of course, accompanied other inventive humor, such as additional meanings for stack overflow, etc. etc.