Someone has made an error in transcribing. The phrase you want is "hush-hush" and it does mean "secret." It's informal and usually a little bit joking or ironic. It is often preceded with the word "very," i.e. "very hush-hush." "Your birthday is tomorrow, and I your husband and kids have been doing a lot of shopping, but it's all very hush-hush and I won't say anything more." "That big building doesn't have any signs on it, and people think that it's some big hush-hush government project." "People who believed in Santeria tried to keep the rituals very hush-hush."

Yes, it should be hush-hush. Keep quiet. "Hash" is a type of food. Also, a type of marijuana :)