Poor John is not just me when I'm feeling sorry for myself! It is a name given to hake that was salted and dried for food. The name first appeared in print in 1585.
Hake was a name given to any of various marine food fishes of the genera
Merluccius and
Urophycis, related to and resembling the cod, but of inferior quality.
"
Poor-john and apple pies are all our fare." Sir J. Harington 1612.
It is unclear whether the John Harrington who wrote that line was the writer of that name (1561-1612) or his namesake, the politician Baron Harington (1539-1613).
If it is the latter there is a wonderful irony in the use of the name since Sir John (he was knighted in 1584 by Queen Elizabeth) was created Baron Harington in July 1603 at the coronation of James I. James then made him guardian of James' daughter, Elizabeth. The high cost of entertaining the Princess ruined him. Poor John!!! As partial recompence Harington was granted a licence by the king to mint the first ever copper farthings.