A person who deems it acceptable to walk around the house while brushing their teeth. This person of course starts in front of the bathroom sink, but maybe was mid-thought just prior to entering the bathroom, he or she then remembers the thought and finds it okay to find the person he/she was initially talking to, sometimes two rooms and a flight of stairs away, to finish the thought with a mouthful of brush and paste.
This is just one potential reason for a person to "brushinwalk" others include: to find something in the house, to play out the last minutes of a videogame, to grab pajamas, ADD and ADHD could all be root causes and outside factors.
Most people who are not brushinwalkers do not like brushinwalkers.
Friend one to Friend two,
"I slept over at my new girlfriends house the other day I found out she was a brushin-walker, I'm not sure how I feel about that. Its kind of annoying."
Friend two back to Friend one,
"Dude, I'm a brushin-walker!"
"Dude, I totally forgot! Sorry man, it's really not that big of a deal."