originally, in the zephyr
system at CMU
it meant adding one point to a reputation.
Later someone coded a 'bot
to actually keep track of the plusplussing (and of course of the minusminusing), and the bot could be queried, leading to ego boosts or great bitterness
sound familiar ...).
I propose this for defining the act of voting up a writeup, because you don't know its reputation before voting. It is a bit like doing, in your programming language of choice,
in a somewhat blind fashion.