What is the origin of the title/phrase "my own private idaho"?

I'm familiar with the movie but don't know why it's called that.

The title is Gus Van Sant's own invention, not a pre-existing idiom he borrowed. But it plays on a real (if a bit obscure) American slang phrase, "one's own private Idaho," which was already floating around before the film — used to mean a person's own isolated, dissociated inner world, an escape from reality into your own head. Idaho works for the metaphor because it's the stereotypical "empty," rural, faraway state — the middle of nowhere.

The most direct proximate source for Van Sant was actually the B-52's 1980 song "Private Idaho," which uses the phrase the same way — a kitschy warning about retreating into your own detached fantasy world ("You're living in your own private Idaho / Underground like a wild potato"). Van Sant borrowed the phrase from that song title/lyric for his 1991 film about Mike (River Phoenix), a narcoleptic hustler who literally and figuratively drifts out of reality into his own dream-state whenever he falls asleep — that private, inaccessible inner world is his own private Idaho.

So: slang phrase → B-52's song → Van Sant's title, each layer reinforcing the same idea of a solitary internal escape hatch from the real world. Fits your subtitle nicely too, as a wink at your own little detached feed of tweets.