Armbrust.

From Middle High German armbrust (12th century, variants armbrost, armborst, armbrast, armbst), from Medieval Latin arbalista, from Late Latin arcuballista, from Latin arcus (“bow”) + ballista (“ancient military engine for hurling missiles”) (compare English arbalest); modified, possibly by folk etymology, but as Grimm notes, the German noun was neuter well into the early modern period which excludes direct interpretation as a compound Arm (“arm”) + Brust (“breast”). The feminine gender became predominant in the 18th century, clearly motivated by popular etymology as a compound in -Brust, also made explicit in hyphenated spelling Arm-Brust. Gottsched (1748) suggested a derivation from Arm (“arm”) + Rüstung (“armament, armour”), in which he was followed by Boeheim (1890).