Andy Bernard had a habit of using the nickname "Big Tuna" for Jim Halpert in John Krasinski played Jim for the duration of the NBC sitcom. Andy's use of nicknames for his co-workers was a major aspect of his personality, but it also connected to his relationship with his peers.
Andy was introduced during The Office season 3 premiere as the Regional Director in Charge of Sales at Dunder Mifflin's Stamford branch. Andy was made the regional manager for two seasons of the series.
When Andy was first presented as a recurring figure in The Office, he referred to Jim as "Big Tuna." It turned out that Jim ate a tuna sandwich for lunch during his first day at the new branch. In addition to Jim's tall physique, Andy thought he was clever with the nickname. It turned out that Andy enjoyed coming up with nicknames for his co-workers throughout his time on The Office so "Big Tuna" was just the first of many.
Other Nicknames Used By Andy In The Office
Like the situation with Jim and his sandwich, Andy used the same method to come up with Ryan Howard's nickname of "Big Turkey." Though Andy had a fondness for nicknames, he wasn't the best at coming up with them. For example, he called Jim "Big Hair" for a brief time as a nod to Big Tuna in addition to just using "Tuna" and "Thin Sliced Tuna." He also tried to come up with aliases for newcomers Clark and Pete, only to use "Fart" and "Plop," respectively. Andy himself went by the self-proclaimed nickname of "Nard Dog," which even spawned into the inspiration behind a tattoo.
As the case with Michael Scott wanting his co-workers to be more like friends during his time on The Office, Andy was obsessed with being accepted by his peers. By using nicknames, Andy figured there would be a sense of camaraderie between the employees at the workplace. It was also a nod to his Cornell University a cappella group who all used special nicknames. Unfortunately for Andy, he was known as "Boner Champ" by his fellow singing group , but he seemed to be pleased with the fact that he had a nickname at all.