[n] an innocent fool, someone easily duped, an ordinary person (lit., "castrated sheep")
(2) a dupe; a sucker; target or victim of a (pecuniary) fraud; to be tricked (Cantonese only)
A person can't learn 1+1 = 2 is a 弱智人士 or 智障人士
羊牯 and 水魚 are slang, for common phrase, a person that is easy to trick is being 好騙
"令人发指" means "outrageous" ...