Seven Deadly Sins: Pride

1.
Pride: From the Anglo-Saxon prut, 'proud'; the Old French prod, 'valiant', 'notable', 'loyal', as in prud'homme; the Late Latin prode, 'advantageous'; and the Latin prodesse, 'to be beneficial'; the compound pro + esse, literally 'to be before'. Pro, 'before', is from the Greek pro, 'before', 'ahead', and akin to the Sanskrit pra-, 'before', 'forward'. In Mark 7:22, huperephania, 'haughtiness', is spoken of as one of the things that come out of a man, thus polluting him. There are two other references to pride in the Epistles.

2.
”To believe in one’s own abilities excessively.”
Pride is a belief in one's own abilities that is so excessive as to deny the recognition of God's grace. When the Seven Deadly Sins were fixed by Pope Gregory the Great, vainglory (a love of honour and glory), which had formerly stood on its own, was folded into pride. But pride is more about loving the oneself.
Thomas Aquinas, the 13th-century religious thinker, summed this up when he said: "inordinate self-love is the cause of every sin... the root of pride is found to consist in man not being, in some way, subject to God and His rule."
In John Milton's Paradise Lost, it's pride that gets Lucifer and his gang of rebel angels hurled out of heaven and into hell: they think they're just as good as God and that they shouldn't have to be subject to his laws. Still, Lucifer maintains his pride after the fall: "Better to reign in hell, than serve in heaven," he says.
Pride is universally acknowledged as the worst of the Seven Deadly Sins because the others are all said to stem from it: without that sense of superiority, you wouldn't have the nerve to flout God by committing the other six. But there's another reason why pride was designated the worst of the worst. In the middle ages, when the Seven Deadly Sins really came into their own, class distinctions were extremely important. If someone aspired to rise above their station, the social order would be thrown into chaos. Designating pride the worst of the mortal sins was a way to keep people in their place.