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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.