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Twerp is an insult commonly used by Team Rocket, usually toward people they do not like. Ash is usually the twerp, (Japanese: じゃりボーイ jari-boy) according to Jessie, James and Meowth. Collectively, Ash and his friends are the twerps (Japanese: じゃりん子 jarinko). The term can be extended to virtually anyone who associates with Ash and isn't on Team Rocket's side. They rarely refer to the twerps by their actual names. In fact, almost every time Team Rocket calls Ash and his friends by their real names is when they are in disguises or when Ash and Team Rocket are forced to work together. This seems to be more out of disrespect than not remembering the names of the twerps.
When they want to refer to Ash individually, they will call him just "the twerp." However, if they want to refer to his friends individually, they will use a variation of this. Their nickname for Brock is "the big twerp" or "the tall twerp." Misty was called "the twerpy little red-head" in Pokémon Fashion Flash. Also, when referring to May, Dawn, or Lyra they will sometimes refer to them as twerpette (Japanese: じゃりガール jari-girl), the feminine version of "twerp" (though it should be noted that the term "twerpette" was not mentioned in the dub until the Advanced Generation series). They also occasionally called Max the "mini-twerp." When May and Dawn first joined Ash on his journey, they were referred to as "the new twerp." Tracey was also once called "the new twerp" and Brock "the old twerp" upon Ash's return from the Orange Islands. When May returned in the Diamond & Pearl series she was referred to as the "Hoenn region twerpette." May was also referred to as "twerp with the bandanna" in May's Egg-Cellent Adventure.
"The rude twerp" and "the fining twerp"
Paul, due to his rude and uncaring personality, is called "the rude twerp," though they have called him "the tall twerp" a couple times, possibly a reference to the fact that he, like all of Ash's rivals, is taller than him. In Strategy with a Smile! Meowth refers to Zoey as "the red-haired twerp." Barry is known as the "fining twerp" because of his consistent threats to fine people for various reasons. Kenny is called "the Contest twerp" in Stopped in the Name of Love! for being one of Dawn's rivals. In Yes in Dee Dee, it's Dawn Ursula is referred to as the "twirly-haired twerpette." In Keeping In Top Forme! Team Rocket calls Marley the "sleepy twerpette."
Additionally, when referring to themselves as Team Rocket, they will sometimes refer to the twerps as Team Twerp in comparison. In the anime, Ash is the one true twerp and all other twerps exist in relation to him. Butch and Cassidy have also called Ash and his friends twerps, and have called Ritchie the same. Attila and Hun refer to Jimmy and Marina as twerps (Japanese: 主人公の少年 shujinkō no shōnen). Thus, it seems to be a common insult by all members of Team Rocket.
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