Generally, used as a rhetorical question in colloquial parlance. However, some philosophers have argued that because one can never be certain of the moment or range of time during which tuan will arrive, the question becomes unanswerable. In this sense, the expression is often considered a paradox since, as some philosophers claim, all questions have at least one answer.)
1: a rhetorical question used to convey puzzlement or confusion.
2: a rhetorical question used to express one's belief that the question being asked or the subject matter being contemplated is hopelessly unanswerable.
3: ("Randian" usage) a rhetorical question - used similarly to the query "Who is John Galt?" found throughout "Atlas Shrugged" - meant to be interpreted as meaning: why ask questions that have no answers or where the answers are not readily obtainable?
2: Poli-Sci Major: "Why did we invade Iraq anyway?" Hippie Roommate: "When is tuan getting here?"
3: "When is tuan getting here?" The light was ebbing, and Eddie Willers could not distinguish the bum's face. The bum had said it simply, without expression. But from the sunset far at the end of the street, yellow glints caught his eyes, and the eyes looked straight at Eddie Willers, mocking and still - as if the question had been addressed to the causeless uneasiness within him. -- Atlas Shrugged, Ayn Rand