Literal Translation
To hold oneself at a quarrel
Actual Meaning
To watch one’s step
Etymology
Carreau’ (= quarrel) was the name for a crossbow bolt.The watchman was always on alert, and was ready to put the quarrel on the crossbow to shoot an ennemy at a moment’s notice. There is also an ‘argot’ (French slang) explanation which says that carre is one’s room, and carreau is the house in police slang. ‘Se tenir à carreau’ would thus mean to remain hidden at home.