This will be short, since I don’t have too many examples. I’d love to learn more, though! You’ll see a lot of these terms when reading material from the Japanese Hidamari fanbase.

デブさん // debu-san – “Fatty”. An unfortunate nickname for Hiro.

へちょ顔 // hechogao – Official term for wideface, as seen on merchandise. 顔 (-gao) means “face”. Hecho supposedly means something like “odd” but it’s a word made up exclusively for Hidamari. So I guess the whole thing is like “odd faces”.

ひだスケ // HidaSuke or HidaSke – Abbreviation for the series name. Most people type out the whole thing, so it’s not very common outside of Niconico.

ナッズ // Nazzu – Nickname for Nazuna.

乃莉っぱい // Norippai – A fan nickname for Nori that started after SPEP01b, the swimming practice episode, aired. Probably inspired by Norippe, one of her nicknames coined by Miyako. Norippai is a combination of Nori and oppai (“breasts”), but also potentially ippai (“full”), referring back to the former.

紗英 // Sae – A very common typo of Sae’s name. The kanji are “gossamer” + “English”, but her name is officially written as 沙英 with “sand” as the first kanji instead. They look extremely similar, so anyone typing fast on a Japanese input keyboard might not notice and accidentally select the first auto-complete option, leading to this error. Other typos and incorrect writings of character names include 吉野家 (Yoshinoya, correct: 吉野屋) 池沢 (Ikezawa, correct: 池澤), and 増子(Mashiko, correct:  益子). Mami’s name (真実) is often written incorrectly as 真美, even in official material! Nori’s name is sometimes jokingly written as 海苔, the kanji used for nori the edible seaweed.

沙英ッコ // Saekko – An affectionate nickname for Sae used by Ume herself on her blog and Twitter. Combination of her name and the same ko used in meganekko, bokukko, dojikko, etc., meaning “girl”. Not used in the series.

ゆのっちペロペロ // Yunocchi pero pero – Basically equates to “I want to lick Yunocchi”. It’s sort of a meme over there. When I asked about it I was (jokingly) told that it’s a “courtship behavior specific to Japanese males and the girls they like”.