Based on this post.
“Clint has given each Avenger a name in sign language. Thor is the letter ‘T’ in the motion of a hammer. Steve is a salute with an “R”. Hulk is an “H” over the bicep, but Bruce is the sign for doctor followed by the letter “B”. Natasha is an “N” being shot from his other wrist like a widow bite. His own name is a “C” being drawn back like a bow. Stark is an “S” taking flight, unless Barton is mad at him, then he fingerspells “A.S.” for Anthony Stark, but accidentally on purpose adds a second “S”. ”
Quoted from the referenced post.