More on Custard Coffin from Infoplease:

• Custard Coffin - Custard Coffin (See Coffin .) Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894 ...
• Coffin - Coffin A raised crust, like the lid of a basket. Hence Shakespeare speaks of a “custard ...
• William Shakespeare: Taming of the Shrew, Act IV, Scene III - No, no, forsooth; I dare not for my life.
• Dictionary of Phrase and Fable: C - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "C"