This term is often used by people who are belligerently drunk. In efforts to describe their obviously non-existent states of sobriety, these people claim to be "sober as a bird." Use of the term is a clear indicator of drunkenness.
Sean: Hey man are you alright?
JP: Yeah man; I'm sober as a bird.
Pajama casual is a term used when working from home and preparing for a video conference. All business above the waist and your pajama pants down below.