Dear word sommelier: Is it better to speak of unceasing devotion or incessant devotion?
Well, I’d say that depends at least in part on how you feel about the devotion: is it commendable or annoying? It also depends to some extent on whether the devotion is continuous or continual. The two words are supposedly synonymous, but we shall see that they do have differences in flavour and usage patterns.
Incessant and unceasing are like long-lost twins separated at birth. Only they’re not quite identical anymore – they’re sort of like Danny DeVito and Arnold Schwarzenegger in Twins. Well, not quite, but they have elaborated differently from the same root. Both come from the Latin verb cessare “cease, stop”, which is in turn derived from cedere “yield, cede”. Both came to English by way of French. But in the case of unceasing, just the verb root was borrowed over and Anglicized as cease in the 14th century, and English affixes were added to make unceasing (also dating from the 14th century). In the case of incessant, the whole French word was borrowed over (with a change in pronunciation) – about 200 years later than cease showed up.
We can note the different sounds and feels of the two words: although both have a scissor-like quality, unceasing starts with the dull, mid-central “uh” vowel and has a “long” vowel peak (written with that more open-feeling ea), while incessant is higher and tighter from start to finish: in is more acute and perhaps pushier than un, the stressed vowel [ε] is a bit lower than the [i:] in unceasing but tighter because shorter, and the following [s] may be thought of as longer (without actually being so) because it’s written doubled. As for echoes, unceasing has sounds of sea, sing, and seizing, while incessant is more likely to bring to mind such words as incense and necessity and perhaps insensitive.
Have those factors helped shape the current shades of meaning and usage of the two words? I can’t say. Actually, it would take a lot of work to come up with even a suggestion of an answer. But we can see what has shaped up. Words are known by the company they keep, and have a look at the kinds of words each one is likely to travel with: unceasing shows up with such as struggle, flow, activity, rain, wind, and demands; on the other hand, incessant shows up with demands, noise, rain, activity, wind… Whaddya mean they look the same?
Actually, while there are overlaps, the emphasis is indeed a little different. Something that is unceasing is typically continuous (like flow), and may be positively, negatively, or neutrally toned (it can be applied to kindness, as by Walter Scott, or a progression, as by Willa Cather, or care or change or toil, as by William Wordsworth). Something that is incessant may be continuous or may be iterative (repeating), and is usually at least slightly negatively toned (like noise; it can be applied to shocks or war, as by Wordsworth, or slashing, as by Sherwood Anderson, or weepings, as by Shakespeare, or pain, as by Christopher Marlowe, or uneasiness, as by H.G. Wells).
Consider, for instance, incessant visits versus unceasing visits. Neither is likely to be very positively toned, though unceasing visits could be; but also, incessant visits means the person is visiting, for instance, every day, without cease, while unceasing visits may mean that or may mean that the visits are of great duration, or both.
And so, too, unceasing devotion may be admirable (though it may be annoying), but it is certainly truly unceasing – as the affixes indicate, is doesn’t cease; it’s continuous. Incessant devotion, on the other hand, is much more likely to be annoying, and has the option of something that happens with great frequency (though that would be more likely with a plural: incessant devotions). It’s a little more removed from the clear literalness of cease because of its lesser resemblance.
For what it’s worth, incessant is also the more common of the two words; if Google Ngrams are to be trusted, it shows up about two and a half times as often as unceasing, and has done so pretty consistently for at least two centuries – although both words had a peak in the mid-1800s and have been subsiding ever since. But, yes, their use has not ceased – which is not to say it is incessant (which implies a very notable frequency).
At a guess, you probably want unceasing devotion – it’s the safer choice in that case anyway (but you can see that you may do better with incessant in some contexts). But there are many other cases where incessant would be the better choice.