In Dorothy L. Sayers’ novel Unnatural Death, the barrister Mr Towkington advises Lord Peter Wimsey to be careful to avoid libel:
‘You are too easily surprised,’ said Mr Towkington. ‘Many words have no legal meaning. Others have a legal meaning very unlike their ordinary meaning. For example, the word “daffy-down-dilly”. It is a criminal libel to call a lawyer a daffy-down-dilly. Ha! Yes, I advise you never to do such a thing. No, I certainly advise you never to do it.’
Dorothy L. Sayers (1927). Unnatural Death, chapter 14. London: Ernest Benn.
What is the libellous meaning of ‘daffy-down-dilly’? The Oxford English Dictionary says only:
daffydowndilly, n. 1. A daffodil; used at first in the generic sense. Still a widespread popular name of the Yellow Daffodil. 2. A shrub: probably the Mezereon, which is still so called in Yorkshire ‘from the slight similarity of the Greek name Daphne with Daffodil’ (Britten and Holland).
And Eric Partridge adds another meaning, equally unhelpful:
daffy-down-dilly. A dandy: ca. 1830-80. Leman Rede in Sixteen-String Jack.
Eric Partridge (1923). A Dictionary Of Slang And Unconventional English, p. 204. London: Routledge.