( BrE , informal ) used to say that you do not think sb's actions are sensible, but that it is their own problem or responsibility :
If he wants to waste his money, that's his lookout.
( BrE , informal ) used to say that you do not think sb's actions are sensible, but that it is their own problem or responsibility :
If he wants to waste his money, that's his lookout.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005