R. Yonah #1: It is one who lacks Midos and Musar - "No'ef Ishah Chasar Lev", "Liknos Chachmah v'Lev Ayin", "v'Shome'a Tochachas Koneh Lev" (6:32, 17:16, 15:32).

R. Yonah #2: It is one who lacks Da'as - "k'Yonah Posah Ein Lev" (Hoshe'a 7:11).

R. Yonah: One who disgraces his colleague is Chasar Lev. This Midah reveals his evil Musar and lack of Da'as - 'do not disgrace anyone, nor dismiss any matter, for everyone has his time, and every matter has its place' (Avos 4:3).

Mesilas Yesharim (Perek 22): If one does not speak honorably to his colleague, rather, with contempt, he is Chasar Lev.

Malbim: Each is learned from its context. There it discusses one who disgraces him without reason. He sins against the laws of Chachmah. Here it discusses if the other disgraced him first. Here he 'sins' only against the laws of Binah - "Erech Apayim Rav Tevunah u'Ktzar Ru'ach Meril Iveles" (14:29). Evilim are Chasar Lev - "ve'Evilim ba'Chasar Lev Yamusu" (10:21). They have no power that rules over their Ru'ach. They do not conduct according to Tevunah to understand a matter properly. They are unsure also about matters of Binah, which obligates delaying anger and not disgracing [in return] immediately.

Rashi: When a Chasar Lev disgraces him, like Sha'ul - "va'Yivzuhu v'Lo Hevi'u Lo Minchah va'Yhi k'Macharish" (Shmuel I, 10:27). R. Yonah - he understands that the one who insulted him is Chasar Lev, it is below his honor to answer him, lest he be equated to him ¹ . Also, it suffices that the other revealed his lack of Da'as, and testified that he himself is Chasar Lev.

Malbim: This is even when he could have answered his disgrace. This is the concept of Charishah ² ; it distinguishes him from his pursuers (who insulted him).