This is the 3553rd most frequent Catalan word.
monumental
Impressive, immense, or relating to a monument.
In this context, 'monumental' is used as an adjective to describe something physically large and impressive.
Vam visitar una catedral monumental a Barcelona.
We visited a monumental cathedral in Barcelona.
Here, 'monumental' is used metaphorically to emphasize the significance or impact of the mistake.
Va ser un error monumental perdre les claus de casa.
It was a monumental mistake to lose the house keys.
In this usage, 'monumental' conveys the notable or memorable quality of the event.