to regret
"Rue the day" means to regret or feel sorrow for something that happened. Example sentence: He will rue the day he decided to betray his friends.
Regretted
The core word is 'rue', or to regret. So ruthless is without regret, and rueful or ruefully is with regret.
It means regret, or fat flower.
In the Hunger Games, the name rue mean a yellow flowers in the pond.
The Rue flower
RUE
lamentable, rue
woe, rue
A six letter word that has the same meaning of 'rue' is regret.
to regret something that has already been done