Final answer:
Andy Bernard nicknames Jim Halpert 'Big Tuna' in S3E1 of 'The Office' due to Jim eating a tuna sandwich on his first day at the Stamford branch.
Step-by-step explanation:
In the episode "Gay Witch Hunt", which is the first episode of season 3 of the TV show The Office, Andy Bernard gives Jim Halpert the nickname "Big Tuna". This nickname is given because Andy observes Jim eating a tuna sandwich on his first day at the Stamford branch. Throughout the series, Andy continues to use this nickname for Jim despite the various situations they find themselves in.