When people say something is as plain as a pikestaff, they mean it is very clear — you cannot miss it, there is nothing to argue about. A pikestaff itself is the wooden pole of an old pike, the long spear soldiers once carried. That stick is simple and unadorned, and that is probably why the saying came to mean “obvious”. Though actually, some scholars think the phrase started as “plain as a packstaff” (a travelling seller’s stick) and later changed shape; you do not need that history to use the saying, but it is why the word feels odd on its own. You will almost never meet pikestaff except inside that idiom. It is slightly old-fashioned, a little literary, and rather British.

（幾乎只出現在 as plain as a pikestaff）再清楚不過、一目了然。不是在說長矛，是在說事情明白得不得了。