Wiktionary
n. (qualifier: film jargon) The first assistant to either the key grip (in charge of camera placement and movement) or the gaffer (in charge of lighting and electrics).
Wikipedia
In a film crew there are two kinds of best boy: best boy electric and best boy grip. They are assistants to their department heads, the gaffer and the key grip, respectively. In short, the best boy acts as the foreman for their department. A woman who performs the duties of a best boy may be called best girl.
Best Boy is a 1979 documentary made by Ira Wohl. The film received critical acclaim, and won many awards including the Academy Award for Best Documentary Feature in 1979.
The film follows Ira's mentally handicapped cousin, Philly Wohl, who at that time was 52 years old and still living with his elderly parents. Ira forces his aunt and uncle to realize that they will not be around to care for Philly forever, and that they must start making preparations for when that time should come. Philly then begins to attend classes in New York City to learn how to take care of himself and become independent.
Philly's father, Max Wohl, dies during the course of the film. His mother, Pearl, died in 1980. Philly is now an octogenarian, and he lives in a group home where he has learned to basically take care of himself. Zero Mostel died in 1977 and the film was dedicated to his memory.
A sequel entitled Best Man: 'Best Boy' and All of Us, 20 Years Later, was produced in 1997. Following the sequel, in 2006, Ira made 'Best Sister' which rounded off the trilogy by looking at the effect Philly's sister had on his current life.
Usage examples of "best boy".
He may be the best boy who ever drew breath and it would not matter.
Here he pulled the remaining yards of the heavy filament around itself and tied it with his best Boy Scout knot.