Shirley Brahms asks for two "John Thomas" cocktails; Captain Peacock corrects her by asking for "John Collins", a cocktail with Gin and lemon. John Thomas is also a slang expression for the male member.
They ordered John Collin's the first waiters real name is John D Collins.
A waiter at the restaurant offers Mrs. Slocombe "a nice apéritif"; apéritif is French for an alcoholic beverage served before a meal as an appetizer. The pronunciation is similar to "pair of teeth", hence her reply of, "Thank you, and they're my own too!"