If something is tenuous it's thin, either literally or metaphorically. If you try to learn a complicated mathematical concept by cramming for 45 minutes, you will have a tenuous grasp of that concept, at best.
Tenuous comes from the Latin word tenuis, for thin, and is related to our word tender. Something can be physically tenuous, like a spiderweb or ice on a pond. We more often use it in a metaphorical sense, to talk about weak ideas. Tenuous arguments won't win any debate tournaments. Synonyms for tenuous, also used physically or metaphorically, are flimsy and shaky.