Green’s Dictionary of Slang

rung n.

In phrases

a few rungs missing off one’s ladder (adj.)

(Aus.) eccentric, crazy.

[Aus]R.G. Barrett Mud Crab Boogie (2013) [ebook] The reason Norton went along with all this craziness was because even if Debbie did have a few rungs missing off her ladder , she had a cheeky sense of humour.