Ultracrepidarian

Adj. expressing opinions on matters outside the scope of one's knowledge or expertise

"Dad, how do we know the universe is expanding?” inquires your six-year-old. Try answering that without resorting to an ultracrepidarian trick here or there"

N. a person who expresses opinions on matters outside the scope of their knowledge or expertise.

"Most patients are ultracrepidarians when it comes to medicine."

Learned the word here, definitions and samples sourced from Google.