In an extremely hot, intense, or rapid manner; with blister-causing heat or speed.
From 'blister' (a fluid-filled bubble on skin from heat or friction) plus '-ing' plus '-ly' (forming an adverb). The word metaphorically extends 'blister' to mean extreme conditions.
When something happens 'blisteringly fast,' you're borrowing the physical sensation of a blister to describe abstract speed—it's how English uses body pain as a poetic measurement.
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