(Obviously this is also from the headcannon of Fundy being Transgender)

When Fundy was about 10 he came out as Trans to Wilbur. Wilbur decided to get him a binder for his 11 birthday which was a bit after him coming out. 

When Fundy wore it for the first time he excitedly ran around to everyone in L'manburg showing them (most knew he was trans by then) by the end of the day everyone in L'manburg and even some outside knew.