I was looking up something else in Webster’s when my eye fell on indagate:
Etymology: Latin indagatus, past participle of indagare, from indago ring of hunters encircling game, act of searching, from Old Latin indu in + Latin agere to drive — more at end-, agent
Date: circa 1623
: to search into : investigate
An intriguing word, but it bothered me that I’d never run into it. So I checked the OED, and the first thing I noticed was the second line of the entry: “? Obs.” If the OED was suggesting it was obsolete in 1900, why on earth is it not marked as such in the 2004 edition of Webster’s? Just to make sure, I googled, and indeed all the hits were from lexicographical sites. To make doubly sure, I googled “indagate the“; at first I thought it was still fitfully in use, because in one of the hits the authors “aim to present and indagate the fundamentals and practice of Plasma Arc Welding,” but clicking on the link showed me that it was (badly) translated from Portuguese. Indeed, indagar ‘investigate, inquire into’ is in current use in Portuguese, Spanish, and Catalan, and I assume essentially all the uses on the internet are from people translating from those languages and assuming, understandably, that since English has the same word, it’s a good translation. I suggest that Merriam-Webster either delete it from their next edition or mark it “obs,” which it most certainly is.
It doesn’t seem ever to have been in wide use; here are the OED citations, a meager crop for three centuries (and note that of the five cites, one is a dictionary definition and two employ a synonym with it):
1623 COCKERAM, Indagate, to search. 1633 J. FOSBROKE Six Serm. Ep. Ded., To indigate and search out the drift and scope of the Spirit of God. 1677 CARY Chronol. II. i, I. xiii. 126 How from them should we indagate the time of his Expulsion? 1829 LANDOR Wks. I. 470/1 We talk of indagating, of investigating. 1867 MUSGRAVE Nooks Old France I. ix. 293 They indagate the history of a hundred and fifty years.