What does it mean when someone is said to ''rust out''?

Read the excerpt from the land. in the late afternoon i did the same, but all the time i was on the stallion, i was aware that mitchell was watching me. he had appeared on the edge of the woods and had just stood there watching ghost wind and me as we went round and round the meadow. finally, on one of our turns past him, he said: "s'pose you thinkin' you a real somebody 'cause you can ride that stallion." i looked down at mitchell and stopped, knowing that despite our understanding, he was itching for a fight with me. now, i don't know what possessed me in that moment to say the next thing i did. maybe i was feeling guilty that because i was my daddy's son, i could ride ghost wind. maybe it was that, but it wasn't out of fear i said what i said. i no longer was afraid of mitchell. "you want to ride him? " i asked. mitchell took a step backward. it was obvious he hadn't expected me to say that. "you know i can't ride him," he said. "your white daddy'd kill me." "you want to ride him? " i asked again. mitchell looked at the stallion, then at me. "so, what if i do? " what intrinsic motivation does the author most likely intend the reader to infer from the passage? paul is motivated by his need to have mitchell praise his riding skills. mitchell is motivated by his need to have paul praise his riding skills. paul is motivated by jealousy and wishes he had free time like mitchell. mitchell is motivated by jealousy and wishes he could ride the horse.