As adverbs the difference between hissingly and missingly
is that hissingly is while hissing; in a hissing fashion while missingly is (obsolete) with a sense of loss.
hissingly
English
Adverb
(
en adverb)
While hissing; in a hissing fashion
* {{quote-book, 1921, , edition=2003 Kessinger ed. citation
, passage=He lingered
hissingly over the word.}}