strengthen
v.v. to make something stronger or more solid. You can use this for physical things, like muscles, or for ideas, like a friendship or an argument.
v. to make or become stronger, more resilient, or more effective. Transitive when describing the act of reinforcing an object or concept; intransitive when describing a natural increase in intensity.
You can strengthen your muscles by lifting weights.
The manager hopes that the new training program will strengthen the team's communication skills.
The recent economic data served to strengthen the central bank's argument for raising interest rates to combat rising inflation.
From rare Middle English strengthenen (14th c.), from earlier strengthen (12th c.), where -en is the infinitive ending. Probably the original form was reinterpreted as strength + -en around the time when the infinitive ending was being apocopated in late Middle English.
The verb is both transitive and intransitive. It often collocations with abstract nouns like 'resolve', 'ties', or 'position'.