The phrase neither fish nor fowl (nor good red herring), also neither fish, flesh, nor fowl, and their variants, mean of indefinite character and difficult to identify or classify. (The signification is similar to that of the late-18^th-century phrase betwixt and between.)

Aided by the alliteration in f, this phrase originated in the ecclesiastical division of food into categories for purposes of abstinence and fasting, the word fish denoting the flesh of fish, as opposed to flesh, denoting the flesh of land-animals, and to fowl, denoting that of birds.

The earliest known instance is from Rede me and be nott wrothe for I saye no thynge but trothe (Read me and be not wroth [= angry] for I say nothing but truth – Strasbourg, 1528), a satire against Cardinal Thomas Wolsey (1474-1530) and the Roman Catholic clergy, by William Roy and Jerome Barlow; according to them, Henry Standish (circa 1475-1535), English Franciscan, Bishop of St. Asaph, played the part of Judas to betray the gospel in England:

– Who played the parte of Iudas?
– The wholy [= holy] bisshop of Saynct Asse
A poste of Satans iurisdiccion.
Whom they call Doctour standisshe
Wone [= One] that is nether flesshe nor fisshe
At all tymes a cōmen lyer.

The English playwright and epigrammatist John Heywood (circa 1496-circa 1578) recorded the phrase in A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue compacte in a matter concernyng two maner of mariages (London, 1546):

She is nother fishe nor fleshe nor good red hearyng.

In The History of Henrie the Fourth; With the battell at Shrewsburie, betweene the King and Lord Henry Percy, surnamed Henrie Hotspur of the North, With the humorous conceits of Sir Iohn Falstalffe (Quarto 1 – London, 1598), by the English poet and playwright William Shakespeare (1564-1616), Falstaff uses the variant neither fish nor flesh when he explains why he is comparing Mistress Quickly, hostess of the Boar’s Head Tavern, to an otter:

– Falstaff: Theres no more faith in thee then in a stued prune, nor
no more truth in thee then in a drawn fox, and for womandood [sic]
maid marion may be the deputies wife of the ward to thee. Go
you thing, go.
– Hostess: Say what thing, what thing?
– Falstaff: What thing? why a thing to thanke God on.
– Hostess: I am nothing to thanke God on, I would thou shouldst
know it, I am an honest mans wife, and setting thy knighthood
aside, thou art a knaue to call me so.
– Falstaff: Setting thy womanhood aside, thou art a beast to say o-
therwise.
– Hostess: Say, what beast, thou knaue thou?
– Falstaff: What beast? why an Otter.
– Prince: An Otter sir Iohn, why an Otter?
– Falstaff: Why? shees neither fish nor flesh, a man knowes not
where to haue her.
– Hostess: Thou art an vniust man in saying so, thou or anie man
knowes where to haue me, thou knaue thou.

The phrase to make fish of one and flesh (or fowl) of another means to make an invidious distinction, to show partiality. This letter to the Editor was published in The Fife Free Press (Kirkcaldy, Fife, Scotland) of 3^rd December 1892:

Sir,—l see that Mr Skinner, draper, Gallatown, a former member of Town Council, has been placed at the bar of the Police Court for occupying the pavement with his goods previous to storage. Now, sir, it is very difficult for shopkeepers to clear off their boxes and packages all at once, or on very short notice, and it is quite well-known that hitherto very great liberty has been allowed in this matter. Although found guilty of contravening the Act under which he was prosecuted, I maintain that ex-Councillor Skinner is no worse a sinner than others who similarly offend—not so bad indeed as many—and why then has he been made a scapegoat of? I hold, sir, that if our authorities have really the law upon their side they should have instituted a general raid instead of making a solitary victim. Why should they pounce upon any one individual to make a test case of, while others, who offend more heinously, are allowed to continue unmolested? This, however, is generally the way in Kirkcaldy, “Fish of one, flesh of another.”