verb/vɛ.je o ɡʁɛ̃/
to be on guard for a sudden, violent gust of wind; to keep an eye on things, to look out for squalls, to keep a weather eye open
“Déjà les biographes veillaient au grain.” — Biographers were already hovering.
Expression arises from maritime use: grain is a sudden, violent gust of wind, accompanied with or without rain; veiller au grain means to watch for, or be on guard for, unpredictable meteorological events that can endanger an ill-prepared crew or damage a frail, poorly sheltered boat.