Why is it called a blowjob?

Why is it called a blowjob? a song by Billy Connolly on Songsofy

Bus drivers in Dublin by Billy Connolly ,
Men shopping by Billy Connolly ,
Gay Bryne by Billy Connolly ,
Reading about yourself by Billy Connolly ,
The 1960s by Billy Connolly ,
Harkins by Billy Connolly ,
Watching reality TV by Billy Connolly ,
Jeffrey Dahmer by Billy Connolly ,
Smoking room by Billy Connolly ,
Cell phones by Billy Connolly ,
Prank call by Billy Connolly ,
Why is it called a blowjob? by Billy Connolly ,
Trying yoga by Billy Connolly ,
Prostate exam by Billy Connolly ,
Visiting the gynecologist by Billy Connolly ,
Aromatherapy by Billy Connolly ,
Going to the beach by Billy Connolly ,
Sleep apnea by Billy Connolly ,
Warn me if I get older by Billy Connolly ,
Computer illiterate by Billy Connolly ,
Shopping for men's shoes by Billy Connolly ,
Big-boned by Billy Connolly ,
Marmite by Billy Connolly ,
Sex scenes in movies by Billy Connolly ,
Signed football by Billy Connolly ,