1. hard; solid; tough
esp. 固い and 堅い for wood, 硬い for metal and stone
すると指先に小さくて固い物が触れる。Then, something small and hard touches my fingertips.
2. stiff; tight; wooden; unpolished (e.g. writing)
そう固くならないでいい。You don't need to be so tense.
3. strong; firm (not viscous or easily moved)
入り口は固く閉ざされている。The entrance is firmly shut.
4. safe; steady; honest; steadfast
私の意志は固いんだ。My mind is firmly made up.
5. obstinate; stubborn
6. bookish; formal; stuffy
まあ、そう固いことを言わないでくれ。Come on, don't be so stuffy about it.