Why it is called 没大没小?
大小 means the seniority or generation.

沒大沒小/没大没小

謂不顧長幼之分。
Regardless of seniority or generation.

不顧長幼之分。形容欠缺禮數，不懂規矩。
Regardless of seniority or generation.
To describe a lack of propriety and ignorance of rules.

典故出處 Origin: 《Journey to the West》
明·吳承恩《西遊記》第23回：“好女婿呀！這等沒大沒小的，連丈母也都要了！”
Good son-in-law!
So impolite!
Even want your mother-in-law too.