Neither fish nor fowl (nor good red herring)
These days, neither fish nor fowl means neither one thing nor the other and dates in this form only from the early 19th century. When it first appeared in John Heywood Proverbs (1546) the full proverb was ‘neither fish nor flesh, nor good red herring’ and was thought to refer to a monastic fasting regimen from medieval times. Most of the original wording, including the ‘red herring’ bit, has fallen away into the largely non-descript saying that evolved during the early 19th century. Red herring in medieval times was the food of paupers and not held in high esteem. During the 17th century red herrings were used to train hunting dogs, which gave rise to another idiom, the infamous red herring.