What are the words and meaning that compose the title of Glasslip? Is it glass + lip?
Is it glass + slip?

The Japanese title is

グラスリップ - it is in katakana, so definitely is a foreign word.

So, the title means lips made of glass, or slipping on glass?
I'm not entirely sure about the meaning behind the name, but I'm pretty sure it is pronounced Glass + lip, mainly due to this image:



Source:
http://animeinspirations.com/threads/is-there-some-hidden-meaning-to-the-title-glasslip.378/