可愛さ余って憎さ百倍
(Kawaisa amatte nikusa hyakubai; “Excessive fondness, hundredfold hatred”)

Definition:

If you had an overabundance of positive feelings toward someone and then something happens that arouses negative feelings, then the resulting hatred is proportionally overwhelming. “The greatest hate springs from the greatest love”; love and hate are two faces of the same coin.

Breakdown:

We begin with the noun 可愛い (kawaii). This is almost universally translated as “cute,” but the literal meaning of the characters is more like “lovable,” and depending on the situation, a suitable translation might be a variety of related words such as “sweet,” “precious,” or even “childlike.” In this case we might render it as “beloved.” Also in this case, the adjective ending is replaced with suffix さ (sa), roughly “~ness,” which turns it into a noun. Next any particle is elided but we have a predicate to 可愛さ’s noun in the form of the verb 余る (amaru), “to be in excess,” appearing in conjunctive form.

The conjoined phrase begins with the adjective 憎い (nikui), also acting as a noun with the help of さ, and in this case the predicate is a noun phrase with copula elided: the number-noun 百倍 (hyakubai), “one hundred times.”

Notes:

Keep in mind that the hatred is not specifically one hundred times as strong as the love; the number is simply a way to express that when one feeling is strong, the other is also strong.

A slightly more low-key version of the saying only raises the hate-stakes tenfold (十倍, juubai). This does not significantly affect the meaning. It’s also perfectly fine to write kawaisa in kana as かわいさ, or to add elided particles; for example 憎さが (nikusa ga).

Mental Health Note: While it makes intuitive sense that being betrayed or attacked by someone you love would leave deeper wounds and fiercer anger than some injury or offense taken from a stranger, love turning to hate is not necessarily the only, or best, response. And a pattern of putting people on pedestals in an excess of 可愛さ, followed by this affection turning to hatred after some error or slight, can be a sign of emotional problems.

Example sentence:

「美人だねって言って来た同僚がいたけど、一緒に飲みに行くのはちょっとって言ったら、私の悪口を言いふらし始めたらしいよ。可愛さ余って憎さ百倍っていうか、負け惜しみっていうか、メッチャ気持ち悪いっ」

(“Bijin da ne tte itte kita douryou ga ita kedo, issho ni nomi ni iku no wa chotto tte ittara, watashi no waruguchi wo iifurashi hajimeta rashii yo. Kawaisa amatte nikusa hyakubai tte iu ka, makeoshimi tte iu ka, meccha kimochi warui–.”)

[“So one of my coworkers was saying that I was really pretty, but after I said I wouldn’t be able to go out for drinks with him, I hear he started badmouthing me behind my back. Call it disappointed love turning to hate, call it sour grapes – it’s just so creepy!”]