This term is used for people who give you that look outside of Walmart for something asinine you've done or look like you are doing.
There are many variations to the look/facial expression but It usually entails nostril flaring a puffed up chin, closed mouth with a frown, and glaring eyes. possibly shacking of the head as if to say 'you bastard'.
The Walmart Face can be given to you buy anyone, anytime, but you usually are doing something to deserve it.
It's when someone gives you a wtf kind of look but they are absolutely disgusted and offended with what ever it is you are doing.
It's more then a look of disgust, It's consumerism mixed with hate, curiosity, and makes you look like someone pissed off at Walmart.
I was driving around and this dude gave me the WalMart Face when I cut him off.
While cutting across old man Wilacer's lawn I knew I was going to be in trouble because he gave us a Walmart Face.