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hard; solid; tough
stiff; tight; wooden; unpolished (e.g. writing)
strong; firm (not viscous or easily moved)
safe; steady; honest; steadfast
obstinate; stubborn
bookish; formal; stuffy
寒い日は地面が堅くなる。
The ground gets hard on cold days.
堅い約束を守ることが大切だ。
A firm promise should always be kept.
彼は堅い性格でとても真面目な人だ。
He has a serious personality and is earnest.