Green’s Dictionary of Slang

undecided, uncertain, ‘neither one thing nor the other’.

1832		Marryat Newton Forster III 95: Dr. Plausible took the lease of a house in a betwixt and between fashionable street.
1876		Besant & Rice Son of a Vulcan I 53: She’s the fool, and he’s the knave, so it’s betwix and between.
1889		Portsmouth Eve. News 5 Sept. 2/6: 178 members of the last French Chamber were thin while only ninety-seve were fat and the remainder were betwixt and between.
1897		Dundee Eve. Teleg. 1 Dec. 3/4: Betwixt And Between [...] ‘Married or single?’ Biddy —‘Nayther. Oim a widdy, sor’.
1906		Spokane Press (WA) 22 Dec. 4/1: A Betwixt an’ Between. I don’t know which I’d rather be, / A rich man’s boy or a poor man’s kid. / [...] for I ain’t nothin’ at all, I guess, / ’Xcept just an old betwixt and between.
1915		Tacoma Times (WA) 23 Sept. 5/4: I have met enough [...] to thoroughly convince myself that there is such a ‘happy medium’ [...] and as many ‘betwixt and between’ men as there are women.
1919		Yorks. Eve. Post 6 Jan. 4/4: ‘What you do you think of my poems?’ ‘Well, they are sort of betwixt and between. They are too sensible for nonsense verses and too nonsensical for sensible verses’.
1922		Dly Ardmoreite (Ardmore, OK) 3 Sept. 9/1: This time, which is a sort of ‘betwixt and between’ season.
1938		Sunderland Dly Echo 17 Jan. 2/3: Our answer [...] must partake a little of the ‘betwixt and between’ attitude.
1951		Yorks. Post 3 May 6/7: You never know where you are with the British climate in Spring [...] That’s why you need a betwixt-and-between overcoat.