before a vowel, brachi-, word-forming element meaning "arm, of the upper arm, pertaining to the upper arm and," from Latinized form of 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."
<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