before a vowel, brachi-, word-forming element meaning "arm, of the upper arm, pertaining to the upper arm and," from Greek brakhion "arm," perhaps originally "upper arm," literally "shorter," from brakhys "short" (from PIE root *mregh-u- "short"), in contrast to the longer forearm.
It is the hypothetical source of/evidence for its existence is provided by: Greek brakhys "short;" Latin brevis "short, low, little, shallow;" Old Church Slavonic bruzeja "shallow places, shoals;" Gothic gamaurgjan "to shorten."
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<a href="https://www.etymonline.com/word/brachiosaurus">Etymology of brachiosaurus by etymonline</a>
Harper, D. (n.d.). Etymology of brachiosaurus. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/brachiosaurus