Yes, it is generally considered rude and unhygienic to put your knife (or any other utensil) in your mouth while dining or in social settings. Utensils like knives are designed to be used for cutting and preparing food, not for putting in your mouth. Doing so can be seen as impolite and may make others feel uncomfortable or disgusted.
Using utensils properly is a sign of good table manners and shows respect for the people you are dining with. When not in use, knives should be placed on the side of your plate or on a knife rest, if provided.
If you need to clean your knife during a meal, it's more appropriate to use a napkin to wipe it off rather than putting it in your mouth. Always try to maintain good etiquette when dining to make the experience pleasant for everyone involved.