眉に唾を付ける
(Mayu ni tsuba wo tsukeru; “to rub spit on your eyebrows”)

Definition:

To be on one’s guard against lies and trickery. A catchphrase meaning that somebody is likely to try to fool or deceive you in some way.

Breakdown:

We begin with the noun 眉 (mayu), “eyebrow(s).” This is marked as a target location by the particle に (ni), and the verb targeting the eyebrows is 付ける (tsukeru), a word with a variety of nuanced meanings but generally along the lines of “to attach.” The particle を (wo) marks the direct object to be attached; it is the noun 唾 (tsuba), “saliva.”

Notes:

This apparently comes from a folk belief that rubbing one’s spit into one’s eyebrows is somehow effective against the trickster powers of foxes and tanuki. (In one explanation, this is because eyebrows anointed with spit appear to be on fire on the spiritual plane, which frightens the trickster away!)

眉 may be expanded to 眉毛 (mayuge), “eyebrow hair.” Alternately, the verb may be replaced with 塗る (nuru), “to paint,” or する (suru), “to do.” The whole phrase may even be contracted to the catchphrase 眉唾 (mayutsuba).

Readers of Japanese may be interested in this 11-year-old comic on somebody’s blog.

Example sentence:

車とか高い物を買う時は、後悔することがないように、しっかりと眉に唾を付けておいた方がいいと思う。

(“Kuruma to ka takai mono wo kau toki wa, koukai suru koto ga nai you ni, shikkari to mayu ni tsuba wo tsukete oita hou ga ii to omou.”)

[“When you’re buying something expensive like a car, I think that you need to make sure to be on your guard against trickery, unless you want to regret it later.”]