On The Office, Andy’s original nickname for Jim Halpert is Big Tuna. Andy explains the name after Jim transfers to the Stamford branch, where Andy notices him eating a tuna sandwich. The deliberately oversized nickname fits Andy’s habit of giving coworkers grand, often awkward labels that reflect his competitive and fraternity-influenced personality.
Andy sometimes shortens the name to “Tuna,” but “Big Tuna” is the recognizable original nickname associated with Jim. It has nothing to do with Jim’s surname, personality, or a hidden reference to fish; the joke is simply that Andy turns one lunch choice into a permanent identity. Jim is visibly unimpressed, which makes the nickname recur as a small source of comic irritation.
Andy Bernard is played by Ed Helms and first joins the American series as a Stamford employee before becoming part of the Scranton office. Other Andy nicknames include “Nard Dog” for himself and “Big Turkey” for Ryan Howard. Jim’s many other labels later in the series, including “Tuna” variations, can make the wording seem inconsistent, but Big Tuna is the canonical answer to the question about Andy’s original nickname.