Definition: Neither one thing nor another.
Origin
For the origin of this phrase we turn to good ole' Henry VIII. It seems his marital problems, led to problems with the Pope which lead to a new phrase for the English language. Charles Earl Funk, author of Curious Word Origins, Sayings & Expressions, has traced the earliest use of the phrase almost exactly to the time of the spat between the Pope and Hank. Originally the phrase pointed a finger at those who abstained from neither fish nor fowl when they should have been fasting. Reading between sixteenth century lines this meant you were neither Roman Catholic or a Dissenter in other words, you were just plain irreligious. Today the phrase has a broader meaning.


