The term "glassiest" refers to something that is the most glass-like or transparent in appearance. In terms of synonyms, one could use the word "crystal-clear" to describe an item that is exceptionally transparent, almost as clear as glass. Another synonym could be "translucent", which implies that light can pass through an object but with some degree of opacity. "Sheer" is another alternative, suggesting a delicate and transparent nature akin to glass. Additionally, phrases like "as clear as glass" or "see-through" can be used in place of "glassiest" to convey the same meaning of extreme transparency.