To mill about is to move around aimlessly in a crowd. Mill about outside.
The aimlessness is the point, since a crowd going somewhere is not milling. People mill about before anything starts and disperse the moment it does.
1phrasemill aboutTo move around aimlessly in a crowd.
“Two hundred people would mill about the square until the doors opened.”
mill about is a phrase.
Practised in A Song of Ice and Fire