This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. (of a container) having the stopper removed

"Musical instruments of the forgotten days — ukukes and nose flutes, and kiokios which are likewise played with one unstoppered nostril."

open   

2-liter    narrow-necked    lenticular    gold-topped    long-neck    antiproton    one-liter    almost-empty    now-open    twelve-ounce