hard, tough, stiff / stubborn

against one's will

strong / strength / powerful / potent

(lengths of time) slightly more than

a male given name

strong, durable, powerful / strong, competent, good / high tolerance, resilient

strength

not always, not necessarily / not completely / not exactly, not strictly speaking, not technically

stiffen / for a person to freeze, for one's expression to freeze

ferromagnetism