The Collaborative International Dictionary of English v.0.48:

Guinness \Guinness\ n. [Trade name.]
   a kind of bitter stout, also called Guiness' stout; as, a
   glass of Guinness.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Guinness
    n 1: English stage and screen actor noted for versatility
         (1914-2000) [syn: Guinness, Alec Guinness, Sir Alec
         Guinness]
    2: a kind of bitter stout