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tightly (holding on); firmly; securely
strongly (built); solidly; sturdily; steadily
properly; well; sufficiently; hard (working, etc.); fully; completely
reliably; dependably; levelheadedly; shrewdly; wisely; cleverly
しっかり握って離さないで。
Hold it firmly and don't let go.
しっかりした計画を立てます。
I will make a solid plan.
しっかり勉強して合格しました。
I studied hard and passed.