In Rome, a big fire?

噓つきは泥棒の始まり
(Usotsuki wa dorobou no hajimari; “A liar is the beginning of a thief”)

Definition:

Someone who becomes capable of lying glibly will also be able to calmly steal or rob. Lies are the first step on a path to more serious wrongdoing. Becoming accustomed to lying also prepares you psychologically for other bad deeds or crimes.

Breakdown:

We begin with the noun 嘘 (uso), “falsehood,” to which is appended the verb つく (tsuku), “to tell (a lie),” appearing in conjunctive form and functioning as a noun: an usotsuki is a liar, and the liar is marked as the topic being discussed with the particle は (wa). The comment is 始まる (hajimaru), “to begin,” also acting as a noun in conjunctive form. The particle の (no) associates this “beginning” with the noun 泥棒 (dorobou), “thief,” “robber.”

Notes:

Note that this saying technically doesn’t apply to social “white lies,” childish fibbing, or even people who lie but feel nervous about it: it focuses on the presumably crime-prone sociopathy that allows someone to lie fluently and without remorse. That said, it’s still commonly used as an admonition against all sorts of lying.

The tsuki may also be written with kanji as 吐き, although this is rare.

Example sentence:

「嘘つきは泥棒の始まりと子供に戒めて五分も経たないうちに、隣の奥さんに罪のない嘘だとはいえ、嘘をついた自分はどうかな。アハハ」

(“Usotsuki wa dorobou no hajimari to kodomo ni imashimete gofun mo tatanai uchi ni, tonari no okusan ni tsumi no nai uso da to wa ie, uso wo tsuita jibun wa dou ka na. Ahaha.”)

[“I warned my kids that lying is the first step toward stealing, and then before five minutes had passed I lied to the lady next door. Even if it just a little white lie, what does that say about me? A ha ha.”]