Why is the output of a flip-flop or latch always referred to as "Q"?

The best answer I have found so far is that the letter "Q" is used to
signify what would ideally be the letter "O".

"O" would be used to designate that it is the Output of the latch, the
flip-flop, or the gate.  However, since the letter O is similar in
shape to the number 0 (zero), Q was used to prevent confusion in
electronic schematics.

This seems to be indicated in the below articles.

http://www.searcheng.co.uk/dcforum/DCForumID4/102.html
http://www.cs.toronto.edu/~hehner/HLCD.pdf

Since I haven't provided any evidence as to the history of the use of
this notation, I'm not sure if you'll consider this a sufficient
answer.  If anything, though, I hope it helps you get what you're
looking for.

Regards.