tenacious

adjective
1
that does not stop holding something or give up something easily; determined
سرسخت, مصمم, پیگیر, ثابت‌قدم, پافشار
2
continuing to exist, have influence, etc. for longer than you might expect
مقاوم, استوار, پایدار, سرسخت, دوام‌آور, ماندگار
a tenacious illness
synonym persistent