deltoidus.

Like deltoid, ultimately from Ancient Greek δέλτα (délta), the triangular fourth letter of the Greek alphabet, and the suffix -(ο)ειδής (-(o)eidḗs, “-like”), the source of English -oid. This suffix has usually been adapted in Neo-Latin as -oīdēs or -oīdeus (giving Latin deltoīdēs or deltoīdeus), but the variant or misspelling -oīdus is attested in some Latin texts. As an English word, the form deltoidus might have been taken from such Latin variants, or may have been created analogically within English by simply combining deltoid with the ending -us (seen in some other muscle names such as trapezius).