Green’s Dictionary of Slang

(middle class) an excl. of congratulation, i.e. splendid! excellent! used (exclusively) by two men.

1909

J. Ware Passing Eng. of the Victorian Era 211/2: Rumbo (Middle-class, 1860). This is an exclamation of congratulation, probably obtained from the gipsies, as amongst them ‘Rumbo’ is a common cry upon the meeting of two men.