( NAmE , informal ) to support or defend sb/sth in an argument with sb
GO TO THE MAT (WITH SB) (FOR SB/STH)
Meaning of GO TO THE MAT (WITH SB) (FOR SB/STH) in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005