Or. ballu.
No definitive etymology can be suggested. In Oelsner 2005–2006:121 and Horowitz 2014:234, fn. 1584 it is proposed to derive bālu from
baˀālu ‘to be large’. This interpretation, although semantically plausible, is hampered by the absence of the “strong aleph” in bālu (one would expect *baˀlu).