I am reading, very slowly, Lig Sinn i gCathú and came to

"ó shacán síl go gimide"

which in the context seems to mean "from asshole to balls" but I can't find either word elsewhere. Did Breandán ó hEíthir make them up?

Full context here:

https://anabhainn.substack.com/p/some-new-vocabulary

sacán síl = "sperm sack"̦ scrotum (balls)
I couldn't find it elsewhere, too.
(FGB: sacán = little sack, síol = seed, sperm, semen)

The second is in FGB
gimide = coccyx (behind the asshole)
elsewhere:
"An gimide atá ag bun chnámh an droma." (Tomás Ó Maille, An Béal Beo)

You're a star: GRMA.