I've occasionally seen "not so much" used at the end of a sentence. For example, Jeff Atwood saying
Some community feedback is useful. Others, not so much.
Doing a symbolhound search for "not so much.", I came across entries such as
- First one good. Second one not so much.
- That worked fine on the development systems, but on a production system, not so much.
- iScroll would be what you need. And unlike jQuery mobile, iScroll works on various devices and on the desktop. jQuery mobile, not so much.
- Again, Safari and Firefox show this, Chrome not so much.
- The lambda I can see the point of. Sending 'fn' to almost-but-not-quite 'f', not so much.
I assume that placing it at the end of a sentence is non-standard English. Is it derived from some quotation or meme?
Urban Dictionary has some entries for it, but they don't describe how the phrase originated, or even use coherent sentences.
(I'm aware of what not so much means when it's used in a more standard manner)