"Overkill" in CoD4 refers to two different things depending on context:
Originally allowed carrying two primary weapons (e.g., assault rifle + sniper). Removed in patch 1.6 due to balance concerns. Not available on Xbox Live or any current version.
On modern Xbox Live (via backward compatibility), the perk does not exist. If you see "Overkill" referenced, it's either historical discussion or slang for a strong setup.
For accurate perk info, consult the in-game create-a-class menu — it reflects the final patched version.