Twinkie defense refers to a defense that was successfully used in a murder case involving San Francisco government officials. In this case, a psychiatrist argued that Dan White, the defendant who shot San Francisco Mayor George Moscone and County Supervisor Harvey Milk, had eaten too many Twinkies (a sugar-rich snack food) to be considered sound of mind enough to possess the conscious intent needed for murder. White's conviction was reduced to manslaughter instead of murder.
Because Twinkies possess a substantial amount of sugar, the defense argued that Dan White was in a reduced mental state. In other words, the defense blamed Twinkies for altering White's judgment. The "Twinkie defense" was considered very improbable, yet it successfully reduced White's sentence. So, now the phrase "Twinkie defense" refers to defenses that seem unlikely to be successful, but that may still yet succeed. In White's case, the defense did not argue that Twinkies made White insane, but that they diminished his reasoning capacity slightly.