malus.

Etymology tree Proto-Indo-European *(s)mel- Proto-Indo-European *-os Proto-Indo-European *(s)molos? Proto-Italic *malos Archaic Latin malos Latin malus Uncertain. Perhaps from Proto-Italic *malos, from Proto-Indo-European *mol-o- (“bad?”), from *(s)mel-. Within Italic, the term is probably connected to Oscan mallom, mallud (“bad”). More broadly, it is perhaps related to Old Irish mell (“destruction”), Ancient Greek μέλεος (méleos, “idle, unhappy”), Lithuanian mẽlas (“lie”), and the first element of Ancient Greek βλάσφημος (blásphēmos, “jinx”). De Vaan, however, considers this etymology uncertain, arguing that this word family consists of "largely isolated words in different IE branches". Earlier associated with Ancient Greek μέλας (mélas, “black, dark”), but support for this is waning. Also compare Avestan 𐬨𐬀𐬌𐬭𐬌𐬌𐬀 (maⁱriia, “treacherous”) and Sanskrit मल (mala, “dirtiness, impurity”).

From mālum n (“apple fruit”). The ending follows the common Latin pattern of having second-declension feminine tree names in -us corresponding to neuter fruit names ending in -um (compare pirus f, pirum and mōrus f, mōrum). The fruit name was borrowed from Doric Greek μᾶλον (mâlon, “apple fruit”), probably ultimately from a substrate root. (Compare Attic μῆλον (mêlon, “apple fruit”), μηλέα (mēléa, “apple tree”).)

Uncertain. De Vaan suggests that the term could derive from Proto-Italic *mazdo-, itself perhaps from Proto-Indo-European *mh₂sdo-. However, De Vaan notes that a comparison with Proto-Germanic *mastaz and Middle Irish maide (“stick”) requires a pre-form *mādos with a shift of d to l. Schrijver, alternatively, connects the term with mās (“male”) The uncertain etymology allows for the possibility of a substrate origin.