The phrase is older than the familiar picture
In English youth slang, “cash money” was used to mean “cool” or “awesome.” So a phrase with a negation works as a reproach: the interlocutor acted not in a friendly way and at the same time not so stylishly. Its tone doesn't resemble a serious moral accusation. It is theatrical disapproval that is deliberately slightly silly.
Short versions of the phrase appear in online posts as early as 2012, while the image of Sayaka Miki in glasses became known later. The early Tumblr post from January 2014 is associated with the drawing; the author of the formula itself is not named in early posts.
Sayaka and the added glasses
The character is Sayaka Miki from Puella Magi Madoka Magica. In the black-and-white drawing she wears large dark glasses; to the right of her face sits a big speech bubble with the phrase itself, and an open palm supports the line from below. So this is not simply a macro with a caption: the words are already built into the panel and sound like a direct address.
The glasses hide the gaze, while the calm mouth and open palm create a deliberately confident delivery. Against this background the silly colloquial slang sounds like a ready-made formula for a scolding. Sayaka doesn't look furious. Rather, the line is delivered with an imperturbable seriousness that makes minor disapproval comically official.
How the format changed
Beyond the original panel, the formula was moved onto other characters: in early 2015 variants the face changed, but the short assessment of the act remained. Later, in 2018, it was picked up again in versions with Godzilla. These substitutions show that the central part became the reaction itself rather than Sayaka's specific plot.
The phrase also lives in chat without a picture. There it works as a short verdict on a minor breach of an unspoken agreement: for example, when an acquaintance takes the last slice of pizza or moves a meeting to an inconvenient time. The built-in bubble in the macro does the same visually: it separates the judgment from a long explanation and gives it a deliberately calm intonation.
The construction itself helps keep the dry tone: “not very” makes the negative assessment intentionally restrained, while “of you” directs the verdict at the person. The formula doesn't explain the consequences of the act and doesn't require a reply. A short line is enough, because the occasion is usually so small that a long explanation would ruin the joke.
Invented captions
“You ate the last slice of pizza that I had labeled with my name.” This is an invented line for the image: the glasses make the complaint almost official, though the reason is minor.
“Moving the meeting to Monday at 8:00 — not very cash money of you.” This invented example keeps the colloquial slang and turns the stern look onto a familiar work rescheduling.