paperknife

UK: /ˈpeɪpəˌnaɪf/

Collins Concise English Dictionary © HarperCollins Publishers::

paperknife / ˈpeɪpəˌnaɪf/ noun ( -knives)

1. a knife with a comparatively blunt blade, esp one of wood, bone, etc, for opening sealed envelopes

WordReference Random House Unabridged Dictionary of American English © 2025

pa′per knife′, 

1. a small, often decorative, knifelike instrument with a blade of metal, ivory, wood, or the like, for slitting open envelopes, the leaves of books, folded papers, etc.
2. the blade of a paper cutter.
• 1800–10