Meaning and usage of 得瑟（嘚瑟）
I believe the core of the meaning is overly expressive with gallantry or proudness. 

Although the most common usages are when guys showing off in front of girls, it can be used in other contexts as well. For example if someone keeps talking proudly about some small deed to his friends, they may react with '你今天怎么这么嘚瑟？', which could be a neutral question, or '你嘚瑟个什么劲儿？', which is a slightly negative critique.