【 コーヒー牛乳 (kōhīgyūnyū) 】coffee milk
銭湯 (sento) public bathhouses became popular around 昭和30年 (Showa 30's) 1955.
Refrigerators were also becoming popular around the same time, but like bathtubs, they were not yet common in average households. Refrigerators, TVs, and washing machines were still a luxury.
Therefore, going to the sento for a hot bath and buying a cold drink afterward, was quite a treat.
So what is the association between a 銭湯 and コーヒー牛乳 ? It was the milk dealers' sales strategy. Since most households didn't have a refrigerator, most people drank milk only once a day from the daily delivery. In order to sell more milk (and milk-related drinks), dealers, catching on to the popularity of bathhouses, started selling to bathhouses that had the newest refrigerator installed.
Sweet milk drinks like コーヒー牛乳 and フルーツ牛乳 or フルーツオレ (fruit-flavored milk) were especially popular. Because real coffee was also expensive back then, (sento entrance fee was 0.15 yen, milk was 0.15 yen, and coffee was 50 yen), coffee-flavored milk was a sweet and satisfying affordable alternative.
Ever since then, having a sweet milky drink after taking a bath at a sento became a custom or culture, a part of the sento experience.
Check out the video that introduces everyday Japanese expressions and phrases with sento vocabulary, including コーヒー牛乳!
Learn more about sentos and onsen here.