【 ワイシャツ (waishatsu) 】dress shirt
The term ワイシャツ is a wasei-eigo that comes from "white shirt" so formerly, ワイシャツ indicated a white dress shirt. Today the term is used more widely covering dress shirts in general (in any color).
ワイシャツ is sometimes written Yシャツ for short — as in Tシャツ for t-shirt — but in this case, it's just a sound replacement.
Fun fact: Shirts ("chemise" in French) were originally a form of underwear (for both men and women) around the 17th century. By the early 19th century, shirts were worn as an outer garment.
Later in Japan, the term "chemise" was adopted as a term for the undergarment for women with shoulder straps. So in Japanese, シュミーズ indicates a "slip" (also called スリップ). Then again "slip" in French is "underwear" for men — and "underwear" in Japanese is パンツ (pants).
So the question is: What is "pants" in Japanese?
Check out the answer, plus how to count pants in Japanese in this video!