Young Spock being bullied in the prime reality
A bully was someone who tormented and taunted others, with the term most often associated with older children "picking on" younger ones, with this relationship common in siblings.
As a child, Spock was the target of bullies in both the prime reality and the alternate reality, due to his half-Human nature. (TAS: "Yesteryear"; Star Trek)
As a child, La'an Noonien-Singh was bullied by other children due to her name and relation to Khan Noonien Singh, being called "Augment" and monster, among other things. (SNW: "Ghosts of Illyria")
When James T. Kirk was a cadet at Starfleet Academy, he was constantly tormented by another cadet named Finnegan who did things like put bowl of cold soup in Kirk's bed, or leave a bucket of water propped over a half-open door. Kirk considered Finnegan to be "positively grim." (TOS: "Shore Leave")
The immortal Flint once referred to his first life as Akharin, saying that he was "a soldier, a bully, and a fool". (TOS: "Requiem for Methuselah")
In 2367, Captain Jean-Luc Picard told his brother Robert that when they were growing up, Robert had been a bully. Robert responded that he had enjoyed bullying Jean-Luc with the statement resulting in the two having a fight. (TNG: "Family")
Later that year, Jake Potts bullied his brother Willie with deadly consequences. Jake tricked Willie into thinking Willie had killed him in a laser duel and Willie, in terror, fled into an alien forest on Ogus II and ate a deadly cove palm. Willie was later rushed to a starbase for emergency medical care. (TNG: "Brothers")
Miss Kyle addressed several concerns about Alexander Rozhenko in 2368, including his bullying behavior. (TNG: "New Ground")
Captain Kathryn Janeway once told Maje Culluh of the Kazon-Nistrim that she didn't like threats or bullies, as she considered him to be a bully. (VOY: "State of Flux")
Philippa Georgiou once told Michael Burnham that when a bully met a bigger bully they crumbled like a cookie. (DIS: "Scavengers")