“In 2006 in an online chat with Hey Arnold! fans, Bartlett revealed that Arnold’s grandpa used his last name throughout the series, almost confirming that Arnold’s last name is “Shortman”. Craig also mentioned that Arnold’s second name most likely is “Phillip”.

In some episodes, other characters can be seen referring to Arnold as Shortman (In the beginning of Arnold Visits Arnie, Helga is telling the other characters their positions in a game of baseball she is quoted saying “Looks like you’re on Shortstop, Shortman.”
In a commercial for Hey Arnold, where Bartlett is interviewed, Bartlett promises that Arnold’s last name will be revealed, and that the audience has been hearing his name throughout the entire series. Grandpa frequently calls Arnold “Shortman.”“

MINDBLOWING.