I agree with Dan and Babyboy on what they've said and it really gets into what it personally means for you.
Growing up, I'd agree that being labeled 'sissy' meant you physically did things or acted in a way that was not masculine. The way you walked/threw a ball/ran/etc basically got you labeled. Guys didn't get called 'normal' it was an unspoken status unless something changed that perception. But if you were weak/wimpy or especially effeminate, then sissy became something that you'd get labeled rather quickly and it wasn't seen as a positive. I know that being teased is pretty normal/regular in all growing up and we regularly called guys sissy or fag as things happened, one weak throw in baseball and you'd get 'razzed'. We didn't truly think it through, it was a jargon of growing boys.
As for guys who dress. Could be just panties, or could be all the way to passing as a woman and all points in-between, it gets a little tricky as some may see it as a big positive being called 'sissy' where others would be very offended. It's meant to be a bit of a humiliating term, one that belittles the person being called it.
From my own personal experience, it was very confusing enjoying wearing lingerie and knowing that men aren't 'supposed' to wear panties and such. If someone saw me in panties, it would not be a stretch to have them call me sissy, even though I didn't feel or act girly. To me, I am a masculine guy that wears, it just so happened it's lingerie. Its the thrill of wearing, the rush and excitement of it that is enjoyable.
What raised the question of 'sissy' as a term for me happened with my first m2m experience. Without all the details, I was more of the 'top' role in that meeting, the man I was with did not wear fem gear, I wore panties/stockings/garters and he commented in a moment of passion that he thought I was a 'sexy sissy'. It shocked me how hot I found that comment. Was I sissy? Really? Afterwards, I felt that the term was what HE thought, not me. He saw me in lingerie and was aroused and that was his feeling. The question that came up in my mind was 'what does being sissy really mean?' It's a personal thing, and his use of the word sparked a thought that sissy can mean various things. It could be a positive or a negative based on context. I liked it. It was a term that meant I'm a guy that dresses, but that's me, and others may despise it. I'm a linebacker build, nobody would ever call me a 'sissy', however, if they saw me dressed in fem gear...that may be possible! LOL.
Since then, I do refer to the times when I'm dressed as either 'dressing' or sometimes as being in 'sissy gear'. For me, it's my ownership of the term. I'm more of a "butch sissy" type. A mans man that wears fem gear. I don't walk girly, don't talk girly and want to be one. But Hell, wearing filly things can be very hot and if that makes my desire to dress seem sissy, then so be it!
As a final note, no way is a crossdresser sissy as a default. Not in my estimation, they could be, it may have no bearing whatsoever.