Hi Pelin. I've heard this used to mean 'put more layers (of clothing) on'. So, perhaps someone is warning a family member to put more on before going out because it's cold outside. Hope this helps! Hugh

'Lawyer up' is a phrase. Maybe you mean that? But we wouldn't say 'don't forget to'

layer up: 1.wear different clothes together, mean mix. 2.put different stuff from button to the top, just like sandwich.