Learn English – swag? And where does it come from

here's one definition of peng

Adj. 1. The best, great, excellent, often heard with reference to drugs. E.g."This weed is peng!" 2. Of a person, very attractive, desirable.

one non-authoritative version of where it came from:

Basically, it means they think you're good-looking/attractive - usually referring to a girl. It can also mean stoned/high on weed/cannabis. (It originates in Nottingham.)

Here's the Urban Dictionary reference:

a word used in areas such as nottingham to descibe something as good. Can also be used to describe someone as good-looking. see also bum

. oh my god, that jacket is proper peng! or Yeh, i'd do him, he's well peng.

I think that stack goes back relatively far in its meaning of "a large quantity". From the book Slang and its analogues past and present written in 1903:

The entry provides the phrase "stacks of the ready" to mean "plenty of money". I think this phrase, in the prevailing years, was shortened to the slang stack, which also took on the meaning of $1000.

Although I am loathe to use this as a source, Urban Dictionary lists that:

one stack = 1 G

That is, one stack is equivalent to one grand which is $1000. The use of stack in this fashion is very much slang, so it is not in more established dictionaries. However, given that Urban Dictionary is a crowd-sourced source which sometimes contains the most current uses of words, and that this meaning of stack is entered twice, I am sure that a stack is equal to $1000.