A person who makes up words.
Extensive training needed over a period of many years/lifetime.
Careers can begin early among the extremely young who are naturally adept at Word-maker-uppery or Word-maker-upping.
Career opportunities limited but with the advent of technical advancements such the interweb and Urban Dictionary, outlets now abound.
"Manny, where are my lallies?" - excellent recent example by four year old Word-maker-upper. Translates as "Granny, where are my sun-glasses?"
"Put your doofer on the knicker-jar" - recent complex example from confused mother; translates as "put your shirt on the clothes airer..."