1.  hard;  solid;  tough
esp. 固い and 堅い for wood, 硬い for metal and stone
2.  stiff;  tight;  wooden;  unpolished (e.g. writing)
3.  strong;  firm (not viscous or easily moved)
4.  safe;  steady;  honest;  steadfast
5.  obstinate;  stubborn
6.  bookish;  formal;  stuffy