Shortened as 'OTT', it pertains to anyone that has crossed the limits of sanity or anger in a conversation, relationship, or situation, or gone crazy to the point of danger.
Its roots come from the trenches of WWI, where anyone who had lost their mind from war trauma and battle would disobey orders and climb over the top of the protective trenches, leading to immediate death.
Used to describe old people, generally with grey hair. Used as a term to express frustration at any old person ahead of you in traffic, a store line, etc.
"I was stuck behind this White Top in Wal-mart counting pennies to buy her Metamucil."