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
Abbreviation of "your" coined by the band Sonic Youth in the 80's, first used in their 1983 release, "Kill Yr Idols". Not necessarily the originators, but Sonic Youth served to popularize its use.
From: Sonic Youth - (I Got A) Catholic Block "You got to earn yr freedom
Get it on my own
I just live 4ever There just is no end"
Guy A(texting to Guy B): Hey, did yo' mom get that root canal at the dentist today? :*
Guy B(replying to Guy A): YR! She thought a root canal was a body of water. :3