Old English, "scorching, hot;" mid-14c. in figurative sense "powerful, strong, ardent;" present-participle adjective from burn (v.)). Meaning "causing excitement" is by 1865, the sense in Burning question (1865), which matches French question brûlante, German brennende Frage. Burning bush is from Exodus iii.2. It was adopted as an emblem by Scottish Presbyterian churches in memory of the 17c. persecutions. Burning-glass is attested from 1560s.
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