Okay, so the reference here seems to be that in WaHH – the only example I've seen so far – when Marisa brought Eirin to the shrine when Reimu was unconscious, when she said "I brought a doctor over", the specific word she used for the "doctor" part is "O-Isha-San".
This is a polite way of saying it, particularly reminiscent of a proper child. The implication here is that Marisa had a good upbringing, and she grew up with these words part and parcel of the manners taught to her.
