In season 1 episode 8 of Kaguya-sama: Love Is War, Miyuki's sister Kei and Chika say to each other 'konnichi sappo'. (A YouTube video showing the scene)
What does this mean?
Anime & Manga Stack Exchange is a question and answer site for anime and manga fans. It only takes a minute to sign up.Sign up to join this community
It is kind of difficult to translate/give this expression a meaning since it is a self-created introduction, but...
Based on the original manga, the wording is こんにち殺法 (konnichi-sappou), a portmanteau of こんにちは (konnichiwa) + 殺法 (sappou), which then is replied with こんにち殺法返し, a portmanteau of konnichiwa + sappou + 返し (-gaeshi).
According to Jisho, sappou means "way of murdering; killing method; way of using a sword" while kaeshi (read as -gaeshi here) means reversal; return. 〇〇-sappou is often used as a killer technique name, while adding -gaeshi to it means the counter for such technique.
Usage of sappou used in this way is kind of rare. 円月殺法 (engetsu sappou) was a popular technique coined by Renzaburō Shibata in his novel series Nemuri Kyoshiro (a YouTube video demonstrating the technique in real-life competition). On the other hand, the usage of -gaeshi is more common, as can be seen in Judo techniques.
So, if trying to give meaning to these expressions, then perhaps "die to/with hello!" and "counter to/with hello!" might be suitable...