Meaning - In disorderly confusion; with reckless haste.
Origin - There are various early uses of pell-mell, which have slightly different meanings. The general sense though is of people charging about 'like chickens with their heads cut off'. See also - helter-skelter.
Sir Thomas North, in his 1579 translation of Plutarch's Lives of the noble Grecians and Romanes, used the term to mean 'in disordered confusion':
"He entred amongest them that fled into their Campe pelmel, or hand overheade."
The first record we have of the term being used with our currently accepted spelling is in Shakespeare's Richard III, 1597:
"March on, ioine brauelie, let vs to it pell mell, If not to heauen then hand in hand to hell."
The expression is derived directly from the French pêle-mêle, which has the same meaning as the English variant. This was an adaptation of the Old French pel et melle (melle means mix; pel may derive from the Old French pesle, meaning to run or bolt).
There is a possible association between pell-mell and Pall Mall, which is best-known now as the name of a street in central London which runs between St James's Street and Haymarket - previously a small alleyway. That name was coined from the name of the game pall mall (a game played with a ring and mallet), which was played in the alley. On the face of it pell-mell and Pall Mall are derived separately and are unrelated. There are early records though, from Samuel Pepys and others, of both the game and the alley being called pell mell. Whether the game was disorderly and confused and the name was coined from that is speculative. It may be that the similarity between the two is merely coincidence, backed up by indifferent spelling.