Showy and ostentatious are both used to describe conspicuous outward display, either designed to attract attention or likely to do so. Showy is the more forgiving word. No one would accuse showy flowers or birds’ showy plumage of being anything but eye-catchingly bold, bright, and colorful. However, showy can also suggest the tasteless flashiness of something “done for show”: showy taste in jewelry. Something ostentatious is blatantly intended to attract notice and to flaunt one’s superiority to others, like an ostentatious engagement ring or an ostentatious lifestyle. Not surprisingly, the word appears very frequently in the phrase ostentatious display, which is usually used of wealth, but can also apply to learning, religiosity, or any belonging capable of being extravagantly flaunted.
Indefensible and untenable describe views, arguments, or stances that are not capable of being defended or maintained against criticism. When indefensible is used of actions or behavior, however, it implies that they are morally unjustifiable, or inexcusable. Untenable, rooted ultimately in the Latin verb for “to hold,” suggests inability to be maintained, supported, or defended. An untenable argument or position on an issue will not hold up, perhaps because they are easily proved to be flawed or unsound, but not only for that reason. New developments in the world may make a position untenable. The common recent usage, untenable situation, means a situation that is incapable of being sustained, borne, or coped with, which is nudging the word further away from being synonymous with indefensible.
In the context of writing and speaking, expand and elaborate (both usually followed by “on”) refer to saying more about an idea, topic, or statement so as to address it more fully or convincingly. Expand on is a slightly looser term–if you say something interesting in class and the professor asks you to expand on that idea, she wants you to say more, but “more” could mean a lot of things–from explaining the idea in more detail to expressing more thoughts about it at large. Elaborate on is used very similarly, although strictly it refers to expanding something that is by implication underdeveloped by adding details to support or develop an idea, point, statement, theme, or argument. Officials are forever declining to elaborate on the terse comments they give to the press, who just want the scoop.