Childlike and childish both mean “resembling a child,” but in very different ways. A childlike person has the positive aspects of a child: innocent, curious, open-minded; whereas, generally, a childish person has the negative aspects of a child: selfish, impatient, prone to outbursts. (Childish can have the positive meaning too, but it’s uncommon.) They’re not quite opposites, but it’s a stark contrast.
It’s strange how two adjectives can form in a straightforward way from a single noun, and end up in such different places. I was thinking about this recently, and being the kind of person I am, I wondered if there were other examples.
Spoiler: Yes.
Noun: sun
Adjectives: sunny, solar
A pretty simple example. A sunny day has a lot of sunshine (literally), and a sunny disposition means you give off lots of sunshine (metaphorically). Solar, by contrast, is more about the sun as an astronomical body: solar flares, solar eclipses. (I guess the moon’s equivalents would be loony and lunar, although loony means something a bit different.)
Noun: star
Adjectives: starry, stellar, sidereal
Continuing with the astronomical theme, we find that starry and stellar are pretty much equivalent to the sun’s sunny and solar. But star has a third trick up its sleeve: sidereal. This word is more focused on the “fixed stars” and constellations collectively, across the heavens, rather than any single star in particular.
Noun: king
Adjectives: royal, regal
Both words are etymologically related to king, or rather, the Latin rex. Royal means “related or connected to kings (and their families)” in a very general way: the royal crown, the Royal Air Force, royal blood, etc. Again, this is loosely analogous to solar and sun. But regal means “having the dignity or magnificence of a king.” It’s more about the feeling or appearance of kingship, rather than a connection to an actual king.
This is a much subtler distinction than, say, childlike vs. childish, partly because the meanings are more similar to each other, and partly because royal can also (sometimes) have the meaning I just gave for regal, and vice versa. Thanks, English!
Noun: water
Adjectives: watery, aquatic
Aqua is simply the Latin word for water, so these adjectives both come from the parent noun in a straightforward way, just like the other cases.
Aquatic covers anything that’s in or on the water: aquatic plants, aquatic sports, the aquatic sciences. But it doesn’t cover the use of water for other purposes — you wouldn’t use aquatic to talk about, say, water for drinking or for watering crops. By contrast, watery means “full of water,” or soaked, or wet, or waterlogged: watery eyes, a watery grave. Here again, the parallel between sun/sunny/solar and water/watery/aquatic is striking, though not 100% perfect.
Also, The Life Watery with Steve Zissou would just be ridiculous.
Noun: tree
Adjectives: treelike, arborescent
That’s right — arborescent is a word. English has two separate words for “resembling a tree,” and that’s all I’m gonna say about that.
There does seem to be a pattern here (putting aside the child and tree examples). Sun, star, king, and water all have one adjective that simply means “related to [noun] in some way,” and another that means roughly “full of [noun] or its attributes.” I don’t think I ever noticed that before, not even when I started writing this post — I only made the connection about halfway through. Neat.
By the way, my latest five posts have been: a love poem, an analysis of presidential approval ratings, a system for classifying degrees of fame, nuclear war, and now, the connection between nouns and adjectives. Can’t say I don’t give you variety!
Brian D. Buckley 
This was a super interesting post! (I’m pretty behind on reading them)
It makes me wonder if it’s b/c older languages (i.e., Greek & Latin) did this and we carried it over into English without realizing it. I wonder if a language historian would know more about why we have this — and I don’t really know enough about other languages to know if they do it a bunch.
Does this happen so much because we love metaphors & comparing attributes of things to other easily understood, prototypical examples?
Don’t know! My guess is that language is sort of chaotic and just evolves in whatever direction it’s useful. In the case of “childish,” it goes all the way back to the Old English “cildisc” — a pretty straightforward combination of the word for “child” and the ending for “-ish.” “Childlike” is more recent, dating to the 1580s, also formed in an unremarkable way (“child” + “like”). Curiously, it originally meant “proper for a child,” and only gained the modern meaning over two centuries later.