Diamond geezer is long established British slang, most associated with London and cockney speech, where diamond suggests someone precious, solid and dependable and geezer is an informal word for a man. The phrase became a familiar compliment in everyday speech, pubs and British comedy, long before short clips turned it into a shareable reaction. Online, creators clip the delivery because it sounds warmer and funnier than simply saying thanks or legend, and it works as a caption, a voiceover punchline or a quick show of respect between mates.