Because I always wear a red jacket, and my friend and I were in the same computer class freshamn year and we were talking to eachother over a site called theotaku.com, and I made a new account and sent him a random pm to scare him. His response-
'I can see you typing it...'
I respond-
'No...dunno what your talking about'
and finally, he said (note): my friend has really bad grammer when it comes to typing:
'I can see you, red jacket...
so, since I was a big FMA nut at the time, I gave myself the nickname redjacketalchemist^^