Buck naked
Definition 1 of 1
Pro Tip 1/3Slang Nature
It's informal and often used in casual or humorous situations, not for formal writing. 
At the party, someone screamed that a guy was running around buck naked.
Pro Tip 2/3Colloquial Usage
Used primarily in North America, other regions might use different terms. 
The movie scene showed the man waking up buck naked on the beach.
Pro Tip 3/3Connotation of Vulnerability
May imply vulnerability or being caught off guard, as clothes often signify preparedness. 
When the fire alarm went off, he had to evacuate the building buck naked.