What does 50 pack year history mean?

smoked 1 pack of cigarrettes a day for last 50 yrs.

Actually it's a little more complicated than what is answered above. Pack-years are calculated by multiplying the number of packs of cigarettes smoked per day by the number of years the person has smoked.

A pack-year is smoking 20 cigarettes a day for one year. If someone has smoked 10 cigarettes a day for 6 years, they would have a 3-pack-year history. Someone who has smoked 40 cigarettes daily for 20 years has a 40-pack-year history.
So it is actually possible for someone to have a pack-year history that is larger than their actual age.