Making a sharp, continuous 's' sound like a snake or steaming water; or showing disapproval.
From Middle English 'hissen,' imitative of the actual sound. The '-ing' makes it a present participle.
Onomatopoeia strikes again—'hissing' sounds exactly like the 's' sound it describes, and both snakes and angry crowds use it to express rejection or threat!
Complete word intelligence in one call. Free tier — 50 lookups/day.