na-adjective, noun
high-handed; domineering; overbearing
Describes a person or attitude that is arrogant, pushy, or acting superior; often used for someone who talks down to others or insists on their own way.
彼の高飛車な態度には本当に腹が立つ。
His high-handed attitude really makes me angry.
上司が高飛車に出
My boss came on strong and overbearing, so the discussion hit a snag.
威張る is a verb meaning to act bossy or throw one's weight around; 高飛車 is an adjective describing the attitude itself.
Originally from shogi (Japanese chess), where 高飛車 referred to an aggressive opening move with the rook (飛車). The figurative meaning of a high-handed attitude developed from this aggressive, forward strategy.