Probably originates from a phrase with 'ehaa "water" and saall, a verb no longer used on its own meaning "falls".
Compare to Santa Ysabel synonyms 'ehaa tuusall and 'ehaa wenaall (lit. "waters are falling").
Can also be said as 'ehaasaall.
Mesa Grande dictionary writes this as hasaall, but the vowel lengths of each syllable in this example appear to be equal.