Simple past tense and past participle of slim. (verb)
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Use "slimmed" in a sentence
"Rather than making any proposals, it simply draws attention to the "various possible options" that have been "mooted", namely a slimmed down commission of 15 members, or a number representing a proportion of the total number of member states, "for example, one half or two-thirds"."
"Surely, not merely because the actor has "slimmed" down."
"But another alternative, say the authors, is that the star "slimmed" to a lower mass, enabling it to become a neutron star."