From Richard II. I feel I’ve read/seen this exchange over a dozen times without ever fully taking in the coldness. There is, by the way, an OED definition of coffer as coffin. The last attestation they provide is 1550 but it’s close enough in time to suggest that the associative flow is dictated by more than just sound.
BUSHY
Old John of Gaunt is grievous sick, my lord,
Suddenly taken; and hath sent post haste
To entreat your majesty to visit him.
KING RICHARD II
Where lies he?
BUSHY
At Ely House.
KING RICHARD II
Now put it, God, in the physician’s mind
To help him to his grave immediately!
The lining of his coffers shall make coats
To deck our soldiers for these Irish wars.
Come, gentlemen, let’s all go visit him:
Pray God we may make haste, and come too late
Extracts 
Ouch indeed.
I’d always read the phrase as a comment on how rich his treasury.
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