In a manner that causes frustration or annoyance; in a way that is maddening or thwarting.
From 'frustrate' plus -ing (forming a participle/adjective) plus -ly (forming an adverb). This is the standard and most common adverbial form of the root word.
The word 'frustratingly' shows how English piles on suffixes—we go from Latin 'frustrare' to add -ing to make an adjective, then -ly to make an adverb, all creating shades of meaning!
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