Ignore him, he's such a grumpy who even complains about young people wearing jeans.

grumpy 是形容词，意思是"脾气暴躁的、易怒的"，常用来形容老年人性格古怪、爱抱怨的特点。这个词来自名词 grump（脾气坏的人）加上形容词后缀 -y 构成。