Smegma. The foul smelling and tasting substance found under the foreskin of a man's penis. Is usually composed of dead skin and other body secretions.
One potential cause of dude butter is permitting ejaculate to incubate and fester underneath the foreskin or within the folds of labia after intercourse. But the latter is extremely irresponsible and gross.
"dude I went on a weekend long camping trip and my dude butter is out of control"