According to the Oxford English Dictionary, the word 'gut' may have its origins in prehistoric times. The dictionary also says that 'guts' was "formerly, but not now, in dignified use with reference to man".
It is not known if the 'catgut' used for the strings of musical instruments was ever the gut of a cat. Usually it was sheep, but sometimes horse or ass.
GUT may also stand for Grand Unified Theory (a theory to unify electro-magnetic force with the strong and weak nuclear forces), The Gdansk University of Technology (in Poland), or the Genito-Urinary Tract.
"I am very proud to be called a pig. It stands for pride, integrity and guts" - Ronald Reagan.