Hi I recommend, www.urbandictionary.com

"Slang" is uncountable because it represents a collection. It is never made into a plural. Also, slang and idioms are different things. Better to write, "What do these (slang) expressions mean?" or simply, "What do these phrases mean?"

A long wish list (you have there): "take the edge off": To take some of the pain away - "He took a swig from the whiskey flask, it took some of the edge off the throbbing tooth." "blind date": I think this isn't as common in China compared to the US. This is a casual date set up usually by an intermediary who knows both parties. Neither of the two have met before. There often is little at stake and there is low expectation of the outcome. Oftentimes, there is a stigma about blind dates - "I can get my own dates, thank you very much."