used to give an important extra reason why sth is true :
His achievement is remarkable; all the more so because he had no help at all.
used to give an important extra reason why sth is true :
His achievement is remarkable; all the more so because he had no help at all.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005