Once just generally meaning confused thoughts but now used to mean when two or more people are talking in an internet chatroom and get two different subjects confused together.
WiL> Have you got it yet?
Tony> I got summat well kewl other day!
Tony> Ill look, BRB...
WiL> Sent it ages ago!
Tony> Its well kewl, a mate sent it 2 me! :D
WiL> He did!? Hang on, weve got Crossed Threads, mate, R U on about the mail I sent U or this other thing?