My cursory search of the phrase to “Keep your nose clean” made references to generally staying away from trouble and evolved from the British way of saying keep one’s hands clean by avoiding masturbation.
I have made my own simple interpretation from my experience being a father of young children. You see, babies don’t know to blow their noses when stuffed so you have to squirt saline solution then suction all the gooey stuff out. Not fun but basic care that must be taken.
So I started using saline for allergy symptoms then expanded to nightly in the dry winter months. For about 3 years no I’ve used it nightly and have not suffered from a head cold or sinus infection since. (since having kids I tend to use hand sanitizer more often so there could be other factors attributed as well.)
My mother used to tell me to keep my nose clean. Now I take the phrase to mean literally keep dust, allergens, dirt, germs, and other ickies out of the last place the air you breathe passes before entering your body.
Thanks for the tip Mom!
P.S. Don’t pay $5 for a little bottle of saline. Shop around and you’ll find it for under $2. Better yet, make your own.