An abbreviation for 'your', first popularized in the 50s and 60s by the Beat generation (ie. Jack Kerouac, Alan Ginsberg), and later repopularized in the indie rock world by Sonic Youth in the 80s.
Contrary to popular belief, the spelling of your as yr did NOT start with Sonic Youth. It actually started with the Beat Generation and the likes of Jack Kerouac and Allen Ginsberg. Kerouac spelled your as yr in his 30 essentials for spontaneous prose.
1. Scribbled secret notebooks, and wild typewritten pages, for yr own joy
Contrary to popular belief, not coined or made popular by indie favorite Sonic Youth. Also, NOT coined by the Beat Generation poets such as Kerouac or Ginsberg. Shorthand for "your" (or "you're), my first encounter with this usage dates back to 1818 in a John Keats letter addressed to John Hamilton Reynolds.