The present participle of barrel; the process of putting something into or storing it in barrels, or moving quickly and forcefully in a straight line.
From barrel (the verb) plus -ing, the present participle suffix. The 'moving quickly' sense comes from the 1800s, likely from the image of a barrel rolling forward unstoppably.
When you see 'barrelling down the highway,' you're using a verb meaning both 'fast movement' AND 'contained in barrels'—a delightful ambiguity showing how barrel became slang for powerful momentum.
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