The phrase is a reference to the young male character Shōtarō ( 正太郎 ?) from Tetsujin 28-go (reworked in English as Gigantor). It can also apply to postpubescent (adolescent or adult) characters with youthful neotenic features that would make them appear to be younger than they are. It refers to a genre of manga and anime wherein pre-pubescent or pubescent male characters are depicted in a suggestive or erotic manner.
Outside Japan, the term is used less often with this meaning. Shotacon ( ショタコン shotakon ?), sometimes shortened to shota ( ショタ shota ?), is a Japanese slang portmanteau of the phrase Shōtarō complex ( 正太郎コンプレックス shōtarō konpurekkusu ?) and describes an attraction to young boys, or an individual with such an attraction.