Aarghh!
I know that, but it’s a moot point because that’s NOT what Trump is saying! He’s saying “big league”, as those of us not hard of hearing can clearly hear.
There are plenty of words in the dictionairy that no self-respecting editor would find acceptable, unless the writer was purposely including colloquial dialogue and wished to establish that the character using the word was somewhat of a dunce.
I dare you to go into a job interview and say “I want this job bigly.”
I might say “in a big way.”
But, bigly would be perfect English grammar, as an adverb modifying the verb “want.”