パンツ一丁

• (short) パンイチ (pan'ichi)

Compound of パンツ (pantsu, “underpants”) + 一丁(いっちょう) (itchō, “one piece”), the latter portion in turn from 一(いち) (ichi, “one”) + 丁(ちょう) (chō, counter). Compare 一丁(いっちょう) (itchō, “(try) one time”, adverb).

• IPA^(key): [pã̠nt͡sɨ it̚t͡ɕo̞ː]

パンツ一(いっ)丁(ちょう) • (pantsu itchō) 

1. (slang) wearing only underpants
   • 2016 October 4, “Kokki no pantsu itchō de F1 kansen Kō no otoko kyu-nin o taiho Marējia [Malaysia: Nine Australian men arrested watching F1 in flag underpants]”, in AFP BB News‎^[1], archived from the original on 7 October 2016, photo caption 1/2:

     マレーシアのセパン・インターナショナル・サーキットで開(かい)催(さい)されたフォーミュラワン（F1）のマレーシアGPで、マレーシア国(こっ)旗(き)柄(え)のパンツ一(いっ)丁(ちょう)の姿(すがた)で観(かん)戦(せん)する男(おとこ)ら

     Marēshia no Sepan Intānashonaru Sākitto de kaisai sareta Fōmyura Wan (F1) no Marēshia GP de, Marēshia kokki e no pantsu itchō no sugata de kansen suru otokora

     Men watching Formula 1 Malaysia Grand Prix at the Sepang International Circuit wearing only underpants bearing the Malaysian national flag

• ノーパン (nōpan): not wearing underpants