JoJo

"Jotaro Kujo... apparently, his friends at school took the "Jo" from Kujo and the "Jo" from Jotaro to nickname him "JoJo"... What a riot !"

- A policeman,

JoJo (ジョジョ) is a nickname shared by each of the main protagonists of JoJo's Bizarre Adventure, derived from the recurrence of "Jo" in their names.

In the text of the manga during Part 1 and Part 2, it is used most frequently in reference to Jonathan and Joseph Joestar.

 Each of the protagonists is also descended from the Joestar Family, and as such all bear the Joestar Birthmark.

Non-Protagonists
The following characters are not protagonists in the series, and some do not even belong to the Joestar bloodline. However, it's notable that their names are able to be shortened as "JoJo":
 *  Geo rge  Jo estar I
 *  Geo rge  Jo estar II
 * Holy  Jo estar-Ku jo 
 *  Shizuka   Jo estar
 *  Geo rge  Jo estar I
 *  Geo rge  Jo estar II
 * Norisuke Higashikata IV (born  Josuke )
 *  Jo sefumi Ku jo
 *  Jo sefumi Ku jo

Non-canon characters

 *  Jo rge  Jo estar
 *  Jo ji  Jo estar
 *  Jo ta Ku jo 
 *  Jo e Ku jo 

Origin
Araki admits that the idea for the name came from his neighborhood's local Jonathan's restaurant where he and his editor would often hold midnight meetings. The two decided the protagonist's first name would be Jonathan, and Araki, using Steven Spielberg's name as a reference, decided the last name should follow the same format, and thus Joestar was decided. Araki mentions that it was risky using this type of name, as using a foreign name for a main protagonist at the time was considered taboo.

Araki also admits that stories such as East of Eden and the TV series Roots inspired him to use a periodical style where the main character changes, but the story continues; although this was not a very Jump-style format for manga at the time.

TRIVIA

 * Giorno Giovanna and Josuke Higashikata are the only main protagonists who were never referred to by the nickname "JoJo" (or the stylized "GioGio") by other characters in canon works.
 * The only time the stylized "GioGio" is mentioned by another character is during the ending of the light novel Purple Haze Feedback wherein Pannacotta Fugo and Giorno Giovanna discuss the events of the story, Giorno tells Fugo to "do him a favor" and call him "GioGio" instead of "Boss", Fugo replies by swearing his allegiance and refers to him as "GioGio".
 * Out of the three protagonists he met, Dio Brando referred to only one of them as "JoJo", that being Jonathan Joestar, possibly out of respect for Jonathan.