pole sisters; women who have slept with the same man
Vulgar slang term for women who have had sexual relations with the same man. The literal meaning is 'pole sisters', playing on 竿 (pole) as a phallic metaphor. Compare with 穴兄弟 (あなきょうだい), the male equivalent.
See also: 穴兄弟
Those two are apparently pole sisters.
I've never heard the term 'pole sisters' before.
Compound of 竿 (さお, 'pole', a phallic metaphor) and 姉妹 (しまい, 'sisters'). The exact origin is uncertain, but it is a modern slang coinage.