turquoisecoast did say it's not a real word, true, but she went on to qualify that it is a portmanteau, which is a legitimate part of our language.
I get that words can annoy us indiscriminately, just like I get annoyed by names that aren't names.
I do think there is more of a distinction between guess and estimate though - the same colour spectrum but not the same shade if you know what I mean.
To me the difference is something like - taking a guess that there will be 500 people at the gig you're going to tonight compared to actually being at the gig, counting the number of people in one row and the number of rows and therefore estimating the number of people from that information.
And if you've ever taught primary age kids, then yes, a guess can absolutely be a stab in the dark :-)