A list of words not taught in class JETs in Tochigi have found useful–for us by us.
唐揚げ（からあげ karaage）: Japanese-style fried chicken (often found at matsuri and in bars)
珈琲（コーヒー koohii）: “Coffee” written in kanji
麺（めん）: noodles (ramen, udon, soba, etc., but not used for Western-style pasta)
助かります（たすかります tasukarimasu）: Literally “I’m saved,” usually a way to express gratitude at someone’s help (also commonly found in the past tense).
ヤバイ（yabai）: slang way to say something is either “cool/awesome” or “awful/crazy” (like “sick” in English)
違和感がある（いわかん iwakan ga aru）:There’s no good English translation, but it’s a good way to say something like, “There’s something ‘off’ about X” or “I’m not entirely comfortable with [someone/an idea/etc.]”–a way to disagree or express discomfort respectfully.
はまる (hamaru): to be crazy/obsessed about something
悩む （なやむ nayamu): to be troubled or worried
からかう（karakau）: Picking on/teasing (someone)
よろしく (yoroshiku): Part of the famous phrase everyone quickly learns as it is a staple in self-introductions: よろしくお願いします. It’s hard to translate into English and is very context-sensitive, but has an overall tone of “work with me here” or “let’s be tight” and can be used by itself casually or part of a phrase formally.
代休 or 振り替え休日（だいきゅう daikyuu、ふりかえきゅうじつ furikae kyuujitsu）: a make-up day off for if you worked overtime
サボる（saboru）: To ditch (class)
貧血 (ひんけつ hinketsu): anemia
化粧室（けしょうしつ keshoushitsu）: The “powder room”
湿気 （しっけ shikke）: humidity
流行る （はやる hayaru）: to become trendy, popular
ゴミ袋（ゴミぶくろ gomibukuro）: trash bag
レジ袋（レジぶくろ rejibukuro）: plastic (or potentially paper) store bag
紅葉 （こうよう kouyou）: the autumn leaves of trees once they’ve turned from green to yellow, orange, red or brown.
鍋（なべ nabe）: a pot or the general type of food cooked in a big pot (like sukiyaki or shabu shabu, for instance)
だいじ (daiji): Tochigi-ben for 大丈夫 (daijoubu or “okay”). The only part of the dialect I’ve heard people of all ages use.
フワフワ（fuwa fuwa）: fluffy
びしゃびしゃ and びしょびしょ （bisha bisha, bisho bisho）: drenched or soaked (ex. if you forgot your umbrella)
“Incorrect” Grammar (for Colloquial Use)
全然いい (zenzen ii): “Totally fine.” 全然 is technically only used with a negative to mean (not…at all), but is used casually in the opposite.
られる→れる (rareru → reru; ex. 食べられる→食べれる): a shortening of the potential conjugation