Tsundere (ツンデレ) is a combination of the words "tsuntsun" (ツンツン), an onomatopoeia for "turning away (to turn one's back on someone, to ignore someone, or to refuse to accept)" which describes an attitude that is "aloof (distant), cold (standoffish), and stuck-up (high-and-mighty)", and "deredere" (デレデレ), a mimetic word for "being lovestruck" which describes an attitude that is cute (having qualities that attract affection)", overly affectionate (extremely and uncritically fond of someone), and desiring lots of close contact and to be spoiled with affection.

A close approximation to how the onomatopoeia "tsun" is often used might be the English onomatopoeia "hmph", which is a sound that expresses "annoyance, disapproval, or indignation" and is usually followed by the person turning their head away to suggest they're too good for the other person. This can be seen when a tsundere makes their trademark pose of turning their head away from their love interest and goes "hmph" to show they don't like them. When applied to a person, tsuntsun refers to "being cold (intentionally) to someone you have feelings for", "keeping someone at a distance", "being dishonest with your feelings", "turning away in disgust", "giving someone the cold shoulder", "acting pretentious (attempting to impress by affecting greater importance than is actually possessed), or "being hard to handle".

To be tsundere is to be dishonest and act distant to attempt to hide one's embarrassment, even though one is secretly in love. They are embarrassed to express affection and unable to be honest. It isn't simply about being moody or selfish, it's an intentional behavior towards their love interest that distinguishes between their true feelings and the public image they create as a result of shyness. They switch between the cold, alienating dishonest attitude (tsun) and the clingy lovey-dovey emotionally open attitude (dere).

The term "tsundere" was created on the Japanese Internet to describe the character Ayu Daikuhji from the 2001 erotic visual novel Kimi ga Nozomu Eien (JP). The term was first coined on August 29, 2002, when a user on the Japanese forum site Suspicious World made a post referring to Ayu's personality as being "tsuntsunderedere" (ツンツンデレデレ) due to the constant switching between her personality. The term tsuntsunderedere began circulating on the site as a popular term and many people on Suspicious World started using the term to refer to her. Eventually, the term spread to another Japanese forum site 2channel where it was soon after shortened to simply "tsundere" (ツンデレ).

While Ayu appears at first to just be a hot-tempered waitress, she is actually an extremely wealthy ojou-sama who is the daughter of the Daikuhji Conglomerate. As part of a study to learn about the lives of ordinary people and experience society she begins working at a family restaurant called Sky Temple. While working at the restaurant she meets Takayuki Narumi, who began working there part-time, and she soon developed a crush on him. Although she always bickers with Takayuki and makes jokes about him, she actually likes him a lot and quickly blushes when Takayuki teases her. She can also often be seen helping Takayuki and she sometimes says things that help him blow away his worries.

Later on she runs away from home due to family problems and goes immediately to Takayuki in tears. Ayu then asks Takayuki to let her move in with him and he agrees. From this point onward she began to be more honest with her feelings and adopted a deredere attitude, giving Takayuki a kiss. The two of them then begin to sleep together and Ayu could be seen blushing and being extremely happy.