a typical saying which originated in the gay community, in cape town south africa, where gay guys substitute women's names for words. in this instance sheila,- which means shit or rubbish - and gail, which means to talk combine to form the meaning 'you are talking shit' or 'don't talk shit!'
gay guy a : you won't believe it! i had sex with brad pitt last night!
gay guy b : Sheila Gail!!
john: i have to use the loo! i need to sheila!
jane: that's a little too much info for me thanks!