In witness whereof, I have hereunto set my hand and caused the seal of the United States to be affixed.

from Abraham Lincoln – The Emancipation Proclamation on Rap Genius

Meaning

He signed it, and placed on it the US Seal, making it all official and stuff.

To help improve the meaning of these lyrics, visit “The Emancipation Proclamation” by Abraham Lincoln and leave a comment on the lyrics box