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ride up(on someone)
1. Lit. [for someone on a horse] to approach someone, riding. I rode up on him and frightened him. I guess I was in the house when you rode up.
2. Fig. [for clothing, especially underpants] to keep moving higher on one's body. I don't like it when my pants ride up on me. I hate it when my underpants ride up.
to slowly move higher, out of position The sweater would ride up in back and she'd have to yank it down - up and down and up and down all day. One problem with the design of seat belts is that the lap belt can ride up over the stomach and injure the passenger.
Usage notes: usually said about clothing
Gradually move upward from a normal position, as in This skirt is too tight and it constantly rides up. [Mid-1800s]
To slide upward across the surface of something and become bunched together. Used especially of fabric or clothing: My sleeves always ride up my arms when I wear this coat. I don't like this brand of underwear—they ride up and are really uncomfortable.