The action of restraining or controlling something; also, a series of curbs (concrete or stone edges) along a street.
From 'curb' with the gerund/present participle suffix '-ing', which can function as both a noun describing an action and a noun for physical materials or structures.
The word 'curbing' brilliantly illustrates how English gerunds shift between abstract action and concrete objects—you can be 'curbing your appetite' while literally walking on curbing, two completely different meanings from the same word!
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