As per the title.

How would one say "weed wacker" in French?

A weed wacker is an infernal little machine which is used for trimming
grass, weeds wherever a lawn mower (*) cannot reach, along a wall for
example.

(*) Lawn Mower = Coup de Grace of course.   :-)

Actually I am serious and I am sure that weed wackers are found all over
France and that every self respecting "jardinier", amateur or pro, owns at
least one.

Merci.