Etymology
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deltoid (adj.)

"triangular, resembling the Greek letter delta," 1741, in deltoid muscle, the large muscle of the shoulder, which is so called for its shape, from Greek deltoeides "triangular," literally "shaped like the letter delta;" see delta + -oid. As a noun, "deltoid muscle," by 1758 (delts, short for "deltoid muscles," is by 1977). Related: Deltoidal.

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Definitions of deltoid
1
deltoid (n.)
a large triangular muscle covering the shoulder joint and serving to abduct and flex and extend and rotate the arm;
Synonyms: deltoid muscle / musculus deltoideus
2
deltoid (adj.)
triangular or suggesting a capital delta, with a point at the apex;
From wordnet.princeton.edu