Obviously this is the slang to use when you know a person is doing anything that it is just stupid and you just feel like saying it, so to point it out you use this slang.
This term has many definitions that may vary upon the situation since it can widely appear in any type of conversation and it will not be misunderstood since it can mean a million things, it is known to even appear in videogames, politics, religion, books, and online chats. You can use it at any moment in the conversation even when a question its being asked.
"Por las mismas nalgas!" is just a variation of the slang that you could also hear in the streets.
"Por las nalgas" is used worldwide, very recognized between black people.
Chris: "oh that night was very exciting, I had a lot of fun, it was 'por las nalgas', Ill tell it was crazy"
Luis: "what a night you must had, it must have being nice for a person so 'por las nalgas' like you. Did you Invite Jean?
Chris: "oh yes, he was the one who got that awesome 'por las nalgas' party going!"
Luis: "Im not surprised!, he loves 'por las nalgas'."