“Ice out” when the ice thaws out far enough to jump into a lake – you jump into the lake.
Family/New England tradition. Within family – a nude tradition. Repeated every year. Was on some level a “Oh haha look the cold doesn’t bother me thing”.
Some nude-related family traditions are usually remembered as bonding moments as this in hindsight is usually quite a vulnerable memory. Informant noted that this has been a thing for as long as they can remember. There’s also the pressure that comes, especially as a child, to prove to your parents that you can handle yourself. In the case, cold is the thing to be handled, and the child can reasonably establish that they are willing to deal with some physical discomfort for the sake of social bonding – not that it is necessarily such an explicit thought process.