WARNING: The following contains spoilers for One Piece Chapter #1021, "Demonio," by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English via Manga Plus and Viz Media.
With the fight between Black Maria and Robin reaching its peak in Chapter #1021 of One Piece, both women go all out and unveil some new attacks that we haven't seen before. Maria, in particular, has some interesting attack names, to say the least. Many feature her own name, which, given how highly she thinks of herself, is rather fitting. But one attack breaks the mold, and in doing so, provides a little insight into what Maria's profession is.
Before Robin defeats her in #1021, Maria captures the giant clone Robin created within her webs and beats it with her knuckleduster. She used this attack before on Sanji in a recent chapter in order to lure Robin to the Banquet Hall, but it wasn't named at the time. This time, however, Maria does say the name of the attack: "Oiran Knuckle."
This attack name doesn't follow the pattern that we have seen from Maria so far. As mentioned, her other attacks include her own name in some capacity, like Mariarson or Maricade. Instead, this one is named after her profession, which is being an oiran. In Japan, oiran is a high-class courtesan skilled in various performing arts, like playing the shamisen or koto, dancing, singing, and being able to hold dignified, high-class conversations with their clients.
Oiran were more highly respected by the community than the average sex worker, known as yujo. Rather than being looked down upon for the profession, they were trend-setters, seen as symbols of femininity before the geisha took this title in the 18th and 19th centuries. Their services were expensive and exclusive, but that was what made them so valuable. Their skills in traditional song and dance and their polite demeanor made them highly sought after by rich clientele.
Maria naming her attack Oiran Knuckle shows that she considers herself to be one of these highly-respected women -- someone of high standing who takes her role as a courtesan seriously. We've seen that she is skilled in traditional musical instruments before in One Piece, which would fit the bill for her being an oiran. Clearly, she doesn't shy away from letting people know about her job. And, beating someone down with brass knuckles in a flurry of punches named after a high-class sex worker -- especially a man -- is more than appropriate for someone like Black Maria.