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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
しっかり握っていてください。
Please hold on tightly.
しっかり勉強しなさいよ。
Study hard.
彼はしっかりした人ですね。
He's a reliable person.