noun

• (sometimes derogatory, colloquial) An effeminate boy or man.
• (sometimes derogatory, colloquial) A timid, unassertive or cowardly person.
  "This was all part of football and if any man was such a sissy he could not stand it, then he had better seek the sidelines."
• (BDSM) A male crossdresser who adopts feminine behaviours.
  "I realised I still held my normal male clothes and dropped them to the floor under the desk, out of the way. […] Would it hurt? Yes, I knew it would from watching videos of sissies being spanked by their dominant mistresses."
• (dated, colloquial) Sister; often used as a term of address
  ""I so glad - so glad the wicked thing not hurt my sissy!" and in that moment of thankful joy the two children embraced each other with fond affection."

adj

• (derogatory) Effeminate.
  "Frontiersmen were never afraid of poetry. It was Big Business with its fear of femininity, it was the eunuchoid clergy capitulating to vulgar masculinity that made religion and art sissy things."
• (derogatory) Cowardly.

noun

• (childish, colloquial) Urination; urine.
  "She has to make. She has to make sissy."

verb

• (childish, colloquial) To urinate.
  "Joan recognized her as the girl whose son had sissied on her pants. She was still dabbing at her pantleg with a damp paper towel."