The original meaning of lag in computing is the delay of packet arrival between client and host, as ping means the delay of "there and back" which is usually 2x lag. I still think that this meaning of the word "lag" was used before anything else as a computing term. Like from the days of IPX/SPX.
This has always been my understanding of the word. Of course there is a more general meaning to it - as others have noted - but that doesn't take into account contextual usage. I reckon that the common misuse of the word arose from the increased popularity of MMORPGs. Dullards would pick up on the use of the word from in-game chat channels and then go on to misuse it outside of their online play.