Ara ara is the Japanese expression, similar to “well, well” in English.
The phrase is often associated with the shotacon hentai genre, which is related to the lolicon kind of hentai, with the exception, that in its center is an underage boy, the shota, instead of the underage girl, the loli.
The exclamation, in memes usually refers to the malicious intents of the female’s sexual intentions, directed towards the boy, who is more often than not is under the age of consent.
The shotacon manga Mother Son Love, created by Cuvie is one such, where ara ara is often encountered, as well as the manga and anime series Aria, where it is used as a catchphrase by a character.
The humorous approach to the term began in 2008, on the message boards of 4chan, where it became a widely used reaction, initially without the sexual context to it, though later on, it got more and more associated with the hentai genre, especially the one-san (elder sister) theme.
The exclamation had spread mostly on sites, like 4chan, Reddit and Tumblr, though we may encounter it on several other places on the web, either in a humorous setting, or a more pornographic one.