A hybrid of misery and hog. The connection of suggesting someone is a miserable pig, is fairly obvious. Although, it is used in a more light hearted and friendly manner, either as a gentle rebuke, or to refer to someone's demeanour, or attitude, rather than an insult.
To someone who is unjustifiable whinging or moaning..."Come on cheer up, don't be such a mishog"
Online about the resident troll. "I see, that Jack's being his normal mishog self"