A person who is prone to outbursts of an excessive nature, usually over petty subject matter.
That fat lady (over there) is a coniptionista. All it takes is the slightest reference to something being overweight and she blows up. Often she gets outraged because people move out of the way as she lumbers down the street and moves into full coniptionista mode by launching into a tirade about prejudices inflicted on fat people, how requiring a fat person to purchase two seats on an airplane was unfair just because they are over 500 pounds, that there is a conspiracy by ruthless corporations to hypnotize people to overeat, how fit people are really fat people with low self-esteem, and so on. A coniptionista is very easily inflamed and readily seeks obscure reasons to explode and cause tension. Full coniptionista mode is achieved by foregoing all formalities and swearing in a manner that would embarrass sailors.