A CodeStench is a really really REALLY bad CodeSmell, an odor so bad that it provokes the retching response, an odor so intense that it permeates the deepest corners of the environment and leaves your clothes smelling bad afterward.
The problem with CodeStench is that it is so intense that it tends to saturate the olfactory organs, numbing them to otherwise-noticeable CodeSmells.
It can also trigger an immediate over-reaction, resulting in immediate mangling of code in an effort to remove said stench ... without the necessary forethought and consideration, or even UnitTests. If you smell something horrendous, do remember to tread carefully. Be a SoftwareMuckraker?:
Theodore Roosevelt coined the term but as fate would have it he became known as one.
My reaction to CodeStenches compared to CodeSmells is the same as my reaction to 'real' stenches compared to smells: they make me want to work somewhere else.