'Y’ is for ‘yegg,’ individuals particularly interested in safes. Their particular interest is getting inside them and taking their insides elsewhere, or if the safe is small enough, taking the entire endeavor elsewhere. Preferably to a workshop with good sound-proofing.
Yeggs are common because safes are common, having been left behind by so many civilizations that thought they were going to live longer. Apocalypses destroyed many possessions, but a sturdy box has outlasted many an owner. Most of the known history (and the better part of the gossiped and unreliable history) are from documents found in safes.
Of course, so are the few functional guns and most of the powerful magic items in circulation. Any worthwhile scavenger has to know how to crack safes, and the really good ones get famous. In The Frontier, Kazh Anzhel gained fame for cracking two gremlin vaults, the only person ever known to perform the feat twice. Being human, he was a pride of the Empire of Gold and Jade, even though he was known to rob them as well. His skill with locks was so great that it was rumored to merge all-chemistry, forging keys that opened doors not only in walls, but in the ground, in the earth, and within gravity itself.
Ninx is one of the main characters of Last House in the Sky.
And no, I didn't make up the word 'yegg.'