Seeing red

Hi, what does it mean when someone says
"i saw red"?

Thanks.
Pueblos.

To see red means to become very angry.

SEE RED - "Become quickly (and perhaps violently) angry. Probably the origin of this thought is the belief that a red flag enrages a bull. In 1901, Mary Harrison writing under the name of Lucas Malet, picked up the expression in 'The History of Sir Richard Calmody': 'Happily violence is shortlived, only for a very little while do even the gentlest persons 'see red.'" From The Dictionary of Cliches by James Rogers (Ballantine Books, New York, 1985).