When Robin said "In haste I struck, but I grieve at leisure", he meant that in hurry to committed an unjust act and he has plenty of time to regret for what he did.
Explanation:
Robin Hood is a popular and renowned legendary hero from the English folktale. He and his stories are featured in many literary works and films.
In one of his story, an incident occured where Robin killed a forester and that's when he said '"In haste I struck, but I grieve at leisure!" By this he meant that, he took action without thinking, in haste, and now he has ample of time to regret his act. What's done cannot be undone, so Robin is sorry for killing the forester.