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firmly; solidly; sturdily; strongly; toughly
彼はがっしりした体格で頼もしく見える。
He looks reliable with his solid build.
がっしりした作りの家は地震に強い。
A solidly built house is resistant to earthquakes.
がっしり組んだ腕は筋肉質で力強かった。
His firmly crossed arms were muscular and powerful.