勝負服(しょうぶふく, shoubu.fuku) is a standard Japanese word that literarily means ‘fight clothes’, where 勝負(しょうぶ) means ‘fight/competition/match’ and 服 (ふく) means ‘clothes’. Originally, it indicates the clothing that a jockey wears during a horse race. However, it also describes your favourite clothes that you wear on a special occasion, especially on a special date with someone (e.g., on a first date, on a gorgeous dinner date, on a Christmas date). There is also anther word called 「勝負下着 (しょうぶしたぎ)」, namely a ‘fight underwear’, which you wear on a special occasion to blow your partner’s mind off.
Related Word 「よそ行き」 and 「一軍 」
よそ行き（よそゆき, yosoyuki） indicates the clothes you wear when you go out or visit someone. Its antonym is 部屋着 (へやぎ, heyagi), which literarily means ‘room wear’, that is ‘loungewear’.
Young people also use ‘一軍 (いちぐん, ichigun)’ to figuratively describe their favourite belongings such as clothes. Originally, it describes the first-tier division of a sports league, especially that of baseball. Likewise, ‘n軍 (n = 2 ,3, 4, …)’ indicates the n-th division of the league, and therefore you can call your irrelevant items ‘二軍(にぐん, nigun)’ or even ‘三軍 (さんぐん, sangun)’ depending on its importance. Occasionally, people reallocate items between１軍 and ２軍 to reflect the change in their taste.
My 勝負服 (favourite clothes) that I often wear on a date got ripped and that made me feel down.
Today I wore a red 勝負下着 (underwear for a ‘special occasion’) but my boyfriend did ‘ドタキャン’ (= cancelled our date at the last minute).
Synonyms: よそ行き (clothes for going out/visiting someone), 勝負下着 (underwear for special occasions)
Antonyms: 部屋着 (へやぎ) loungewear
Related Words and Phrases: 勝負 (しょうぶ) match, competition, fight; 服(ふく) clothes,