Both "to freak" and "to freak out" are slang, and you can use either of them. "Freak out" is a phrasal verb. I understand the "out" part to mean "fully/completely", but that doesn't apply to every phrasal verb that uses "out". It depends on the context.

Thank you so much to all three of you, :)

"Freak out" = think of it as shorthand for "freak out of his mind". For example: He is going to freak out. = He is going to freak out of his mind. Or: He freaked out. = He freaked out of his mind.

You could say both of these, which mean the same thing: "He will freak out." "He will freak." Note, however, that the second has multiple meanings, while "freak out" has only one specific meaning. Perhaps the slang includes "out" to be more precise.

'Freak out' is mostly used together and you have to accept that. Its a slang. It means to become 'anxious,scared ,upset' etc. I freaked out on seeing the snake in my garden. You freaked me out when you told me that you are going for a divorce. Example of usage of 'up' Lets split up the prize money. My boyfriend broke up with me last month. Whatever u say is incredible, I doubt you made it up. Will you be able to make it up for the wedding?