Esp. 固い and 堅い for wood, 硬い for metal and stone

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

to harden; to freeze; to strengthen; to solidify; to make (a fist); to tramp down (snow, dirt)

to put together; to collect; to gather; to consolidate

to make secure; to stabilize; to settle down; to strengthen (belief, resolution, etc.); to establish (evidence)

to fortify; to reinforce; to support

In the form に身をかためる

to wear for a specific purpose (armor, coat, etc.)

to swear; to resolutely vow; to sincerely promise

to tie tightly; to fasten

to hold a bow fully drawn

formal; stiff; strict; ceremonious

steady; sound; stable; safe (e.g. business); reliable; trustworthy; solid

solid; strong; sturdy; durable; stout

straight-laced person; stubborn person

solid; strong; firm

nucleus; backbone; mainstay; key figure

Nouns which may take the genitive case particle 'no'

medium-level; mid-level; middle-ranking; midsize

main body (of troops); crack troops; select troops

Baseball

center field; centre field; center fielder; centre fielder

athlete competing in the middle-number match in a team competition, i.e. second in 3-on-3, third in 5-on-5 (kendo, judo, etc.)

Colloquial

prudish; strait-laced; boring; square

somewhat hard; somewhat firm; fairly hard; on the hard side