The canonical use of -ish is as a suffix meaning "approximately," as in bluish, tallish, sixish, or even hungry-ish. This is the definition—the only definition—that you'll find in Merriam-Webster, which notes that -ish derives from the Old English -isc, of Germanic origin, which in turn is related to similar such suffixes in Dutch (-isch) and Greek (-iskos).
For centuries now, -ish has been rather promiscuous in English, attaching to a wide variety of words and even phrases. Take the following architectural observation, cited in the Oxford English Dictionary, from an 1894 article in The Daily News, a now-defunct London newspaper (of which Charles Dickens was briefly the editor!):
Some huge pile of building, generally much more Queen Anne-ish than the houses of Queen Anne's own time.
As a word by itself—which is to say, not as a suffix—ish means more or less the same thing: kind of, thereabouts, in a way. And imagining how it broke free to become syntactically stand-alone isn't hard. The word "hungry-ish," say—as in, I guess I could eat. I'm hungry-ish.—often comes out more like "hungry [brief pause] ish." From there it's a short leap to:
Are you hungry?
But while it's quite common for new words to be formed by adding prefixes or suffixes (editorialize from editor, anti-nuclear from nuclear), or even by re-casting a portion of a word that hadn't before been thought of as an affix (snowmageddon based on armageddon, chocoholic based on alcoholic), it's exceedingly uncommon to form a new word by keeping the suffix and discarding the rest. But that's exactly what ish did, a process that Mr. Verb, a blogger who writes about language and linguistics, calls degrammaticalization.
In fact, ish is atypical even among its uncommon cohorts, such as ism and ology, which both refer to a general class to which the more specific exemplars belong. In other words, nationalism and communism are kinds of isms. Zoology and biology are kinds of ologies. But are hungry-ish and tired-ish kinds of ishes? Not exactly.
Instead, as described by the linguist Stefanie Kuzmack, ish refers back to a particular idea, often in the form of an adjective, that was previously mentioned. You could repeat the idea, but you don't have to:
Would you say that movie had a happy ending? Yeah, ish. [happy-ish]
Is everyone excited? I am—ish. [excited-ish]
Will the weather be nice tomorrow? I think so, ish. [nice-ish]
And ish doesn't even need to be near the word that it refers to. For example, in the following sentence ish modifies "hobbies," not "skating":
My hobbies include painting, photography, documentary film, and skating(ish). [meaning skating is hobby-ish]
This freestanding ish feels relatively new, but is it really new, or only new-ish?
The earliest citation in the OED is from a 1986 review of So Long on Lonely Street, by the American playwright Sandra Deer, which John Peter—then chief drama critic of The Sunday Times in England—called, "… one of those neatly crafted middle-brow plays which, because they have a pleasantly happy ending (well, ish), might make people think that they've been handed a soft option. Not at all."
In the 1990s, ish makes an appearance in several novels by English and Irish-born writers, including Cycle of Violence by Colin Bateman, who employs ish as a single-word answer to a question in a bit of dialogue:
'Davie Morrow. Trust Davie Morrow.'
'You know him?'
'Ish. He's a regular across the road. Thick as shite, like, but as liable to give you a hidin' as look at you.'
There aren't any examples on this side of the pond until 2002, when the New York Times quoted Jeremy Langmead—a Brit!—who had just been named editor of the "obsessively stylish London-based magazine Wallpaper":
And so it is fair to ask: Does he have a Wallpaper apartment?
Mr. Langmead, speaking by telephone from London, hesitated. 'Ish,' he said, employing the international shorthand for slight hedge. His East London home, then, is obsessive-ish.
Note that the Times seemed to believe back then that the word required an explanation, probably because it did. In fact, it was shortly after, in 2003, that ish began to really take off. That's the year it pops up for the first time on Urban Dictionary.
But ish isn't universally acknowledged yet. Merriam-Webster, Random House, and whatever dictionary Google is drawing on contain only the suffix version, and my spellcheck is still underlining it in red. Of course, even when a word is a decade or two old, it can be slow to take root in the reference books, some of which are notoriously reluctant to add what sounds like slang. But there's no doubt that what may have originated as a Britishism is now firmly fixed in American English too. To not include it in our dictionaries is to pretend that it's not a word. Which means that, with respect to ish at least, we're so early '80s. Ish.