Green’s Dictionary of Slang

a phr. used to justify a practice, usu. in the workplace, that would otherwise be condemned, abandoned etc had it not been established over a long period; usu. an old Spanish custom; earlier an old Southern customcit. 1952 is a pun.

1952		W. Winchell On Broadway 10 Sept. [synd. col.] ‘Franco demands more American millions’ (It’s a gold Spanish custom).
2002		Warwick Boar 9 May 🌐 If you are hungry and you visit a tapas-bar, don’t be shocked when you see olive pits, dirty napkins and toothpicks littering the floor because it’s an old Spanish custom to just drop all disposable things.