Bucky Barnes/The Winter Soldier (Sebastian Stan) has had a considerable evolution throughout the Marvel Cinematic Universe. He went from a supporting character in Captain America: The First Avenger to being the star of his own Disney+ series, The Falcon and the Winter Soldier. Barnes has been through a lot within the franchise, originally becoming a supporting villain in Captain America: The Winter Soldier when it was revealed that he was working for HYDRA as a brainwashed super soldier and carrying out various assassinations in their name while the villainous organization was busy dismantling S.H.I.E.L.D. from the inside.
The origins of his brainwashing were later revealed in Captain America: Civil War, where Bucky became a central figure in the ideological war between Steve Rogers/Captain America (Chris Evans) and Tony Stark/Iron Man (Robert Downey Jr.). This was also where viewers learned about the specific sequence of words that trigger Bucky's brainwashing. But does it have any deeper meaning? Let's investigate.
The opening scene of Civil War shows Bucky being brainwashed by a Russian HYDRA soldier who utters a series of words from a red book, known as the Winter Soldier Book. The film's main villain, Zemo (Daniel Bruhl), recovers the book and uses Bucky to help carry out his mission to divide the Avengers and avenge his family after Sokovia was destroyed in Avengers: Age of Ultron. The words that Zemo recites are as follows: "longing," "rusted," "seventeen," "daybreak," "furnace," "nine," "benign," "homecoming," "one," and "freight car." When all of these words are uttered in this order, Bucky obeys any command he is given, no matter how violent.
In this brainwashed state, Bucky has committed several acts of violence in the name of either HYDRA or Zemo. These acts include killing both of Tony Stark's parents, almost assassinating Nick Fury (Samuel L. Jackson) during the events of The Winter Soldier, and attacking Tony Stark and several others while being detained in Berlin during the events of Civil War. The latter incident, as well as the revelation about Tony's parents, were some of the final nails in the coffin for Steve and Tony's relationship and led to several intense battles between the two heroes, with Bucky getting caught in the middle of them.
Bucky's trigger words came up again in the fourth episode of The Falcon and the Winter Soldier, "The Whole World Is Watching." In a flashback, Ayo (Florence Kasumba) assisted Bucky in his quest to deprogram his brainwashing. Eventually, Ayo recites the trigger words to him and he was stunned when he showed no signs of hostility towards her or obedience to her every word. Now fully cured, the trigger words are discarded and Bucky completes his transformation into a true hero.
There has never been any definitive meaning behind these specific words and why they were chosen to activate Bucky's brainwashing. However, that hasn't stopped people from coming up with various theories as to how Bucky relates to these words, connecting them to Bucky's past and present in various ways. There were even theories that the word "homecoming" was meant to be a reference to Spider-Man: Homecoming, since Peter Parker made his MCU debut in Civil War and his upcoming solo film was releasing the following year.
That theory was quickly shot down by Civil War's writers, who also didn't confirm if the words were chosen because they have any specific meaning to Bucky. But with Bucky now cured of his brainwashing problem and the character ready to move on from this dark chapter in his life, speculation about the words now seems much less important.