Origin unclear.
Speculation: perhaps from Modern English (though obsolete, and dialectical)
atter, that from Old English
aettor poison, pus
(1) (Wiktionary--
atter), plus Greek suffix
eu (or
ev) good, well
(2), with an adjectival ending
-a. (English
atter poison is not related to
adder, the snake, incidentally.) This seems semi-plausible, given that the author of the genus was English.