Green’s Dictionary of Slang

(US) applied to an outside lavatory or privy and denoting the number of seats available, e.g. one-holer, two-holer etc.

1945		in Yank 6 Apr. 5: A brand-new eight-holer had been constructed [HDAS].
1953		T. Runyon In For Life 17: I stole an occasional watermelon [...] or tipped over a two-holer on Halloween.
1965		L. Bruce How to Talk Dirty 14: I reviewed it [...] in my ‘reading room’ — a four-holer.
1987		(con. 1967) E. Spencer Welcome to Vietnam (1989) 152: I’m een the four-hole sheeter at Charlie Med taking a sheet.
1998		L. Hasselstrom et al. Leaning Into the Wind 75: We had a two-holer. My aunt and uncle had the best outhouse. They had a four-holer. Each hole was a different size.