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

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ad. In a mysterious manner

ad. In a mysterious manner.

In a mysterious manner; by way of expressing or implying a mystery; obscurely: as, he shook his head mysteriously.

ad. in a cryptic manner

"Weinstein had at one point indicated plans to release the film on Blu-ray, but the title mysteriously disappeared from their release calendar."

seriously