A close examination of "indagate" reveals that it's a rather uncommon word. If we delve into the past, we discover that it first appeared in an English dictionary in 1623. Probing further, we see that its synonym "investigate" was already a hundred years old at the time. Despite the fact that our search turns up the derivatives "indagation," "indagator," "indagatory," and "indagative," we see that none of these words was ever used as widely as "investigation," "investigator," "investigatory," and "investigative." If we hunt for the etymology of indagate, we sniff out the Latin verb indagare ("to track"), which often referred, as did Latin investigare, specifically to tracking done by hunting dogs.