This is the chilliest day I've ever experienced, even a thick coat can't keep the cold out.

chilliest 是形容词 chilly 的最高级形式。chilly 意思是"寒冷的、凉飕飕的"，由名词 chill（寒冷）加上形容词后缀 -y 构成。chilliest 表示"最寒冷的"。