(According to my file, this is the 404th kotowaza post!)

幽霊の正体見たり枯れ尾花
(Yuurei no shoutai mitari kareobana;
“[I] saw the ghost’s true form: dry pampas grass”)

Definition:

When something seems frightening at first but turns out to be benign after closer inspection – or when a person is so full of fear or doubt that they end up jumping at shadows and seeing threats where nothing is actually amiss.

Breakdown:

We begin with the noun 幽霊 (yuurei), “ghost,” “apparition.” This is followed by the associative particle の (no) in its possessive function, then by the possessed thing: the noun 正体 (shoutai) – literally “correct body,” but in this case best rendered as “true form.” Then without any particles that is taking the preceding noun phrase as its direct object: the verb 見る (miru), “to see,” in conjunctive form as simply 見 (mi), with auxiliary verb たり (tari), which expresses that an action has been completed. This is likely also appearing in conjunctive form (also たり, conveniently enough) so that it can connect to what follows: a noun phrase with all further verbs  or parts elided.

The final noun phrase begins with the verb 枯る (karu), “to wither,” “to dry out,” in prenominal form as 枯れ (kare), modifying the noun 尾花 (obana), the brushy frond at the end of a stalk of pampas grass. (Literally this one is “tail flower,” which I rather like.) Anything else, such as a copula, is elided.

Notes:

This saying is attributed to the Uzuragoromo (『鶉衣』, literally “quail clothing,” figuratively “ragged clothing”), an Edo-era collection of poetic writings by Yokoi Yayū. Apparently the original form of the phrase used a 化け物 (bakemono), literally “a thing that changes form” but often translated as “monster,” but that this was changed to a ghost through memetic association. See also 疑心暗鬼.

Incidentally, the ゆ (yu) entries of the three big iroha karuta sets are either 油断大敵 or an excessively obscure phrase, 幽霊の浜風 (yuurei no hamakaze), which describes someone as listless as a ghost that has had to put up with the buffeting of a wind blowing in from the sea. While I was looking for more information on that, though, I found this saying, and the rest is history.

Example sentence:

「本を怖がるなんて、正に幽霊の正体見たり枯れ尾花じゃない？可哀そうに」　「あいつらがススキを怖く思ったから燃やしたと言っても、花園まで燃やしてしまったらいくら何でも許せないぞ」

(“Hon wo kowagaru nante, masa ni yuurei no shoutai mitari kareobana ja nai? Kawaisou ni.”  “Aitsura ga susuki wo kowaku omotta kara moyashita to itte mo, hanazono made moyashite shimattara ikura nandemo yurusenai zo.”)

[“Being afraid of just a book, isn’t that exactly a case of seeing ghosts in every rustle of the grass? I feel sorry for them.”
“Even if they say they did it because the ‘grass’ scared them, there’s just no way I could forgive someone who burned down the whole damn garden.”]