What it means
と思いきや presents the preceding proposition as an expectation that is immediately overturned by the following statement. It conveys a striking contrast such as “I thought A, but unexpectedly B” or “contrary to expectations.” The expression is commonly used with plain predicates; nouns and na-adjectives generally take だ before it. The following clause should clearly reveal the unexpected reality or result.
Usage note
と思いきや has a literary, formal, and often dramatic tone, so it is more common in writing, narration, headlines, and emphatic descriptions than in ordinary conversation. It is not normally replaced by a simple polite ending; politeness is expressed in the following clause instead.
Example sentences
静かな催しだと思いきや、会場は開始時刻になると熱気に包まれた。
I thought it was a quiet event, but when the start time arrived, the venue was unexpectedly enveloped in excitement.