1Zoology. Of a fish: having ventral fins below the pectorals. Of ventral fins: below the pectorals. Now rare.
2Anatomy and Zoology. Located below, or on the underside of, the forelimb or arm.
3Anatomy. Located below a brachium (arm-like structure) of the brain.
Origin
Mid 19th century; earliest use found in Encyclopedia Americana. From post-classical Latin subbrachialis under the arm from classical Latin sub- + brachiālis.