Sometimes you run across a word that you never heard before, and when you look it up, you can’t figure out how to make the same sentence without it:

…Her bare knees caught and compressed my wrist, and slackened again; and her quivering mouth, contorted by the acridity of some mysterious potion, with a sibilant intake of breath came near to my face.

I seriously doubt I could make the same sentence with such perfect economy.

sibilant |ˈsibələnt|
adjectivePhonetics

(of a speech sound) sounded with a hissing effect, for example s, sh .
• making or characterized by a hissing sound: his sibilant whisper.

ORIGIN mid 17th cent.: from Latin sibilant- ‘hissing,’ from the verb similar