In a nutshell, the name Sean is sometimes used to secretly mock a child who was an accident (accidentally conceived) then becomes an example of social isolation or being a "loner" in later life.
"I know man, what a Sean."
"That guy over there is such a bender."
"Totally, no wonder his parents named him Sean."
emily: good i met this guy, but i keep texting him and he wont reply.
amelia: he totally seaned you.