A documentary that is misleading, often using slight of hand and statistics, purposely designed to fox* you.
*Fox = To trick or fool by ingenuity or cunning; outwit. To baffle or confuse.
Note: It is often incorrectly thought that 'foxumentary' came about as a disparaging term for documentaries seen on the Fox network. This is not the case, and all networks my put out foxumentaries. It is merely a coincidence that the fox network is perceived by many as putting out nothing but foxumentaries.
"That anti-environmentalist film was a foxumentary."
"That's not a fair and balanced! It's a foxumentary."
"That program on Bush's legacy was a foxumentary"
The 'documentary' on the fox network last night was a foxumentary" (pun - how apt)