I used this for about a year after I started teaching to mean “opposite word” before one of my students reminded me that in mathematics it actually means “not equal to” and that there is a perfectly acceptable mathematical symbol for opposites in a line with an arrow at each end. 
Not only does that also make much more sense, it also frees up the top symbol to be used for the very useful meaning “not equal to”, for example when you want to emphasize that words are not the same.
I just assumed that it was something I’d misunderstood or badly remembered from my TEFL course (not for the first time) and switched to using arrows for opposites as he suggested and never looked back. Until this very week that is, when I spotted an equals mark with a line through it to mean “opposite” in an EFL textbook I was browsing.
So, I am right in suddenly realising that it wasn’t just me? Is this particular abuse of mathematics actually quite prevalent in TEFL? If it is, when where and how did it start? Do you do it too? If so, do you promise to stop? Is that enough questions? Are you sure?(etc)
TEFLtastic 
Since you posted about simplifying the error correction key when giving written feedback, I’ve tried to make my symbols even simpler. I’ll be steering well clear of this one thanks!
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Wow, I do believe that would make you the first person to ever do something I’d suggested!
You’re right that it wouldn’t work in a writing correction code. I use it on the board during presentations and corrections, e.g.
“Warm (equals sign with cross through it) hot” to show they aren’t the same
“Warm (line with arrows at each end) cool” to show that they are opposites.
Writing the word “code” in “writing correction code” above made me realise what a telling word it is in that description!
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Wow . . . I never realized how much geometry would effect the rest of my life! So . . . I’ve been doing the arrows for opposites . . . not so much the crossed out equals sign, but I still also use the three dots for therefore (in my own notes, not on the board.) Does anyone else still do that? Crazy! Thanks for posting, this was interesting!
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Oh shit, me too! Guess I’ll have to stop pretending to my students that I’m a scientist and go back to using the ‘traffic sign’ of a double-headed arrow.
BTW, Alex, is there a sign for things such as ‘this teacher is bored’ or ‘why don’t you all go to the library and let me paddle about on the web’?
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