ELF He may have stripped to his hairy arse in ''Old School,'' but Will Ferrell hadn't encountered real humiliation until he dressed up like Santa's adult-size helper in November's ''Elf.'' ''Putting on tights is the clothing form of being naked,'' laughs the former ''Saturday Night Live'' goofball, who we see learning about the Christmas spirit before heading to the New York City home of his biological dad (James Caan). So did the actor find it hard getting into his dim-bulb persona? ''Wearing the outfit and having New Yorkers yell 'Nice tights!' helped,'' says Ferrell. ''You'd get catcalls constantly.'' Mee-oww!