Hey, friend, although this question is indecent, I am also very interested^_^
I am from Chengdu, and the word "日" is often used, so I will say I'm familiar with it.
There is a saying: There is a word in ancient Chinese characters that describes that meaning, which is also pronounced "日", but the top of this character is "入" and the bottom is "肉". Together they are pronounced "日" sound. Now this word is no longer used, so it is replaced by "日".
However, I personally think this statement is not necessarily accurate. After all, it is difficult to find the ancient Chinese characters for "入" + "肉". We might as well understand it from another angle:
The character "日" is used in Sichuan and Chongqing. In the past, when I heard senior elders describe what it was like in the old society when Pao Ge steered a target (that is, a gangster), they also said this in their words, which proves that this usage has been around for a long time, at least during the Republic of China. At the same time, this word often appears in common people's swear words and cannot be used in court. At that time, few poor people could read, so I thought "日" had nothing to do with those ancient books that explained Chinese characters.
So, if illiterate people invent this usage... I think they can only rely on pictograms to understand it.
You might as well use your imagination... Look, isn’t this word very vivid? -_-b
Personal opinion, for reference only!