A state of being unable to remember seemingly simple things such as keeping appointments and returning phone calls.
While occasionally a natural condition for some people, it is most often a temporary state triggered by excess stress and/or a lack of sleep. It should not be considered an accurate indicator of a person's intelligence.
I didn't mean to stand you up at the coffee shop last night. I got a bit moonbrained and completely forgot we had plans.
To let one's beard grow long, preferably to mid-chest length. This is in reference to the famous beards worn by Billy Gibbons and Dusty Hill from ZZ Top.
My beard? I like to top it off before winter so I don't have to wear a scarf.