Julia 0.3, 22 bytes

c->s->c&95∉[s...]&95

uppercase is a long word.

How it works

f(c)=g(s)=c&95∉[s...]&95

f(c)=                     Define a function that takes a character c and returns...
     g(s)=                  a function g that takes a string s and...
          c&95                Compute lo bitwise AND of c and 95.
                              This casts the character c to uppercase.
               [s...]         Yield the list of the characters of the string s.
                     &95      Compute lo bitwise AND of each chararacter and 95.
                              This casts the characters of s to uppercase.
              ∉               Return a Boolean, signaling non-membership.

Julia 0.3, 22 20 bytes

c%s=c&95∉[s...]&95

uppercase is a long word.

How it works

c%s=c&95∉[s...]&95

c%s=                Redefine the binary operator % so it takes a character c and
                    a string s and...
     c&95                Compute lo bitwise AND of c and 95.
                         This casts the character c to uppercase.
          [s...]         Yield the list of the characters of the string s.
                &95      Compute lo bitwise AND of each chararacter and 95.
                         This casts the characters of s to uppercase.
         ∉               Return a Boolean, signaling non-membership.

Julia, 22 bytes

c->s->c&95∉[s...]&95

uppercase is a long word. Try it with Coding Ground.

How it works

f(c)=g(s)=c&95∉[s...]&95

f(c)=                     Define a function that takes a character c and returns...
     g(s)=                  a function g that takes a string s and...
          c&95                Compute lo bitwise AND of c and 95.
                              This casts the character c to uppercase.
               [s...]         Yield the list of the characters of the string s.
                     &95      Compute lo bitwise AND of each chararacter and 95.
                              This casts the characters of s to uppercase.
              ∉               Return a Boolean, signaling non-membership.

Julia 0.3, 22 bytes

c->s->c&95∉[s...]&95

uppercase is a long word.

How it works

f(c)=g(s)=c&95∉[s...]&95

f(c)=                     Define a function that takes a character c and returns...
     g(s)=                  a function g that takes a string s and...
          c&95                Compute lo bitwise AND of c and 95.
                              This casts the character c to uppercase.
               [s...]         Yield the list of the characters of the string s.
                     &95      Compute lo bitwise AND of each chararacter and 95.
                              This casts the characters of s to uppercase.
              ∉               Return a Boolean, signaling non-membership.

Julia, 22 bytes

c->s->c&95∉[s...]&95

uppercase is a long word. Try it with Coding Ground.

Julia, 22 bytes

c->s->c&95∉[s...]&95

uppercase is a long word. Try it with Coding Ground.

How it works

f(c)=g(s)=c&95∉[s...]&95

f(c)=                     Define a function that takes a character c and returns...
     g(s)=                  a function g that takes a string s and...
          c&95                Compute lo bitwise AND of c and 95.
                              This casts the character c to uppercase.
               [s...]         Yield the list of the characters of the string s.
                     &95      Compute lo bitwise AND of each chararacter and 95.
                              This casts the characters of s to uppercase.
              ∉               Return a Boolean, signaling non-membership.