oathable

From oath +‎ -able.

oathable (not comparable)

1. Capable of having an oath administered to one: fit to be sworn.
   • c. 1605–1608 (date written), William Shakespeare, “The Life of Tymon of Athens”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act IV, scene iii]:

     You're not oathable, Altho' I know you'll swear Into strong shudders th' immortal gods