Scarred for life is a common phrase, usually referring to a psychological trauma that will remain with someone for the rest of their life.
It may alternatively refer to a physical injury, specifically a cut on the skin that will never heal completely and will leave a scar tissue.
The exact origin of the phrase is not known, although “scarred for life” has been documented as such in the 1850’s.
It’s a common expression, appearing in both the common lingo as well as medical jargon, especially as advancements were made in psychology and psychiatry.
Today it is common knowledge in child rearing that a kid must be protected from all negative experiences that may inhibit their growth and leave them “scarred for life”.
The expression is often used humorously to the common experience of finding one’s parents having sex, as is presented by the 2013 entry on the subject from Urban Dictionary.