Definify.com

Webster 1913 Edition

adv.

With a hissing sound.

adv.

With a whistling sound.

hissingly ‎(comparative more hissingly, superlative most hissingly)

While hissing; in a hissing fashion
- 1921, Aldous Huxley, Crome Yellow, 2003 Kessinger ed. edition, ISBN 0766137503, page 272:
  He lingered hissingly over the word.