His apartment was super clean, and he had a pet dog—major green flags.

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The slang term "green flag" originates from the physical green flag object, symbolizing a sign of approval. With the rise of social media in the late 2010s, the term began to shift more toward describing an individual's qualities. People typically use the term "green flag" to describe positive qualities in others, particularly in friendships, romantic relationships, or to describe themselves.

For example, one might note that their biggest green flag is how positive they are with kids, signaling their nurturing and caring nature. In relationships, a green flag could be someone who communicates openly and honestly. In friendships, it might be a person who consistently supports and respects boundaries. On social media, many post their list of green flags when looking for a relationship or friendship to pursue. This contributed to its popularity and, therefore, constrained usage of the phrase.