This was originally written as a ballad by Phil Lynott, before the guitarist Brian Robertson and drummer Brian Downey rocked it up.
The lyrics, which were written by Lynott, tell of a man warning his loved one not to believe his words as he cannot be relied upon. The chorus sums it up: "Don't believe a word/For Words are only spoken/Your Heart is like a promise made to be broken."
No, don't believe me if I tell you
Not a word of this is true
Don't believe me if I tell you especially if I tell you
I'm in love with you