Simple past tense and past participle of firm.(verb)
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Use "firmed" in a sentence
"The cuttings must now be dropped three inches from each other in the little furrows, the ground levelled over them and firmed, which is best done by walking on a board laid on the covered drill, or else by the use of a garden roller."
"The compost must be carefully 'firmed' into the pots, but no severe pressure should be employed, or the roots will not run freely."
"Huge pickles, still smacking of the brine that "firmed" them, are offered in lieu of fresher acids."