What is the origin of Gian's motto 'What's yours is mine, and what's mine is mine'?

Hello! You've really hit upon the essence of Doraemon with this question. This famous line is practically synonymous with Gian.

"What's yours is mine, and what's mine is mine."

Original Japanese: 「お前の物は俺の物、俺の物も俺の物」 (Omae no mono wa ore no mono, ore no mono mo ore no mono)

This classic quote, of course, comes from the original manga Doraemon by Fujiko F. Fujio!

Where does this quote come from?

You might find it hard to pinpoint the exact "first time" this line appears in Doraemon, as it wasn't born in just one specific episode. More accurately, it's Gian's (Takeshi Goda's) catchphrase and personal motto.

A Regular Occurrence, Repeated Often: In the everyday stories of the manga and anime, whenever Gian wants to "borrow" (i.e., take) comics, toys, or snacks from Nobita or Suneo, he confidently shouts this line. It became his standard opening line when bullying his friends.
Core to His Character: This line is truly a stroke of genius by Fujiko F. Fujio. In the simplest, most childlike way, it perfectly captures Gian's image as the "unreasonable bully." Hearing it instantly conjures up the image of that large-framed, imposing figure with a fierce expression.

Why is this quote so famous?

Extremely Distinctive Characterization: There's no more direct way to portray someone's domineering nature. This "my rules are the rules" logic, while infuriating, is highly dramatic and memorable.
A "Trigger" for Plot Development: Many episodes start with Gian taking Nobita's things. Nobita runs home crying to Doraemon, who then produces a gadget... You could say Gian's famous motto is the starting point for countless adventures.
The Flip Side: Loyalty: Interestingly, in some movies or touching episodes, Gian uses this phrase to express his loyalty. For example, when Nobita faces real danger or is bullied by outsiders, Gian might step up and say, "Nobita's trouble is my trouble!" At these moments, this domineering line transforms into a promise meaning "I've got your back," instantly making Gian's character multi-dimensional and endearing. Despite his bullying nature, deep down he still considers Nobita and the others "his people."

So, in summary, this motto originates from Doraemon itself. It was designed by the author as Gian's signature line to shape his character, runs throughout the entire series, and has become a classic anime quote because it perfectly showcases the character's complexity.

Hope this helps!