Loli is used for prepubescent looking, or actually prepubescent girls, mostly in the anime/hentai community.
There are three main kinds of loli; true loli, who is between four and fourteen years old, teen loli who is between thirteen and seventeen years old, and legal loli, who is over eighteen but still looks prepubescent.
Origin of the term
The term loli comes from the Japanese word lolicon. It refers to Lolita complex, from the famous novel of Russian author Nabokov, Lolita, in which a man is having a romantic relationship with an underage girl.
Spread of the term
Loli is an expression mostly used by weebs and otakus, who are – for some reason – aroused by underage girls.
It is quite widespread in hentai and fanfiction, in which fans of anime and manga characters channel all their deprived and dark fantasies.
The very first Urban Dictionary entry was added about loli in 2004, though a more precise one has been added in 2019, which translates the attitude of many people about it; “The FBI is on their way.”