"weak, effeminate men", from sis(ter). Singular 'cissy'
Why the spelling with c- (sometimes) in British English?
e.g. in
- He said his name was Ashe with an ‘E’ he added quickly, and Leamas knew he was lying. He pretended not to be quite sure that Leamas really was Leamas, so over lunch they opened the parcel and looked at the National Insurance card like, thought Leamas, a couple of cissies looking at a dirty postcard. (John le Carre - The spy who came in from the cold)