According to the latest and most accurate etymological research conducted by Fiorenzo Toso (Piccolo dizionario etimologico ligure, Zona, 2015), the term bellin [beˈliŋ] “male sexual organ” is simply a derivative of bello, borrowed from varieties of the Po Valley area with the original meaning of “toy” and specifically “rattle” (from which the meaning of “male sexual organ” later entered Liguria). For this reason, the most coherent spelling of the word (and its derivatives) is with a double consonant (bellin). The same applies to the word’s derivatives (bellan [beˈlaŋ], bellandi [beˈlaŋdi], bellinata [beliˈnaːta], bellinon [beliˈnuŋ], belliniximo [beliˈniːʒimu], etc.).

That said, spellings with a single consonant (belin, belandi, belinata, etc.) are also considered fully acceptable, due to their widespread use and to the status of belin as a linguistically emblematic word.