Throughout history, many cultures have had a personification of luck: Tyche, Fortuna, Lakshmi, St. Cajetan. RNGesus is the modern incarnation of the same tradition, though more akin to the secular Lady Luck. I first encountered the term in the mid 2000s, but with the similar Random Number God as an antecedent, I suspect it's been around for far longer. As you can probably gather, it's an irreverent portmanteau of RNG and Jesus, and is a particularly common term in tabletop gaming and RPGs. Many players will offer a tongue-in-cheek prayer to RNGesus before a particularly important roll or draw, or praise Him after a timely bit of good fortune.
RNGesus, who art in game,
Hallowed be thy name.
Thy fortune come, thy will be done, in die as it is in deck.
Give us this day our daily rolls.
And forgive us our luck, as we forgive the lucky.
And lead us not into misfortune, but deliver us from chance.
For thine is the topdeck, and the crit, and the rare drop.
For ever and ever.