an expression of courtesy that is used to greet an unexpected guest who drops by to visit to acknowledge them for the trouble they took in coming, similar to phrases like "what brings you here?"
Definitions (粵典–words.hk)
語句
行錯、蕩失路。謙稱自己低微,對方行錯到先至會去自己果度。 (lit) to get lost and go to a wrong place; (fig) a humble phrase meaning it is by coincidence that the visitor bumps into the speaker's place
乜咁錯蕩啊?
乜咁错荡啊?
mat1 gam3 co3 dong6 aa3? How very kind of you to have come!
Definitions (Wiktionary)
verb, Cantonese, humble
to wander to the wrong place; to bump into my humble abode
乜你咁错荡啊?
乜你咁错荡啊?
mat1 nei5 gam3 co3 dong6 aa3? What brings you to my humble abode?