I'll tell you what

English

(This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium.)

Synonym of tell you what
- c. 1596–1599 (date written), William Shakespeare, “The Second Part of Henry the Fourth, […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC, [Act I, scene i]:
  My lord, I'll tell you what;/ If my young lord your son have not the day,/ Upon mine honour, for a silken point/ I'll give my barony: never talk of it.