"Prudence" is a principle in accounting and finance. It means "acting with due care". In concrete terms, here it means doing proper credit analysis before you lend.
"Imprudent to lend" here means "the results of the credit analysis show that the bank should not lend money to the applicant; the possibility of deliquency and default is high." It would be reckless to lend.
It is not wrong to say "unwise" or "careless", but you should try to learn the more sophisticated explanation above.