Google nGrams is a useful tool for this sort of question, it allows you to search across a large corpus of books and plots results over time.

The word "stealth" has been comparably common with the words "badger" and "teapot" but much less common than the word "fish". Actually, "stealth" used to be even more frequently used (within Google's book corpus) in the early 19th century than it is today.

What is notable is the relative change of "stealth" and "by stealth". That tells us that between 1800 and about 1975 the word "stealth" was mostly used in the phrase "by stealth", as can be seen by the way those two lines track each other. Since the late 1970s there has been an increased use of the word "stealth", but not as much of an increase in the stock phrase "by stealth". I would put forward a hypothesis that this has been caused by the military uses, supported by the coincidence of the increase in uses of terms like "stealth technology" and "stealth fighter" that use "stealth" as a modifier rather than a mass noun.