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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.
しっかりした計画を立てよう。
Let's make a solid plan.
しっかり食べて元気を出して。
Eat well and get your energy up.