kabgnals, n. — Green’s Dictionary of Slang

kabgnals n.

also kabac genals

[the word itself is backslang spelt backwards and its use – spoken very quickly – is a coded way of asking, ‘Do you understand backslang, and shall we use it for this conversation?’]

backslang.

1859		Hotten Sl. Dict.
1909		J. Ware Passing Eng. of the Victorian Era 161/2: K. A. B. G. N. A. L. S. (Mystic). The letters of back slang (less the needless ‘c’), and uttered rapidly to indicate that this mode of conversation will be agreeable to speaker. Another form is Kabac genals.
2001		J. Burkardt ‘Back Sl.’ Wordplay 🌐 kacab genals: back slang.