autisticemmalyn.com/autistic-arms/
I used to do this a lot more as a child. One of my memories is of being criticised for the way I held my bag so I forced myself not to do it and I now wrap my hands around each other at waist height instead.
I'm trying hard to not mask at all these days and I've noticed that I still t rex with my dominant arm/hand when I'm walking around looking for something. Also, if I do force my hands to my waist, I'm more likely to rock or sway from side to side.