impenetrable /im-penˈi-trə-bl/

1. Incapable of being penetrated
2. Impervious
3. Inscrutable
4. Occupying space exclusively (physics)

impenetrabilˈity noun

• red shifts
• redshire
• redshirt
• Red Shirt
• redshirted
• redshirting
• redshirts
• redshort
• redskin
• redskins
• red snapper
• red snappers
• red snow
• red spider
• red spider mite
• red spider mites
• red spiders
• red squirrel
• red squirrels
• redstart
• redstarts
• redstreak
• redstreaks
• reds under the bed
• reds under the beds