Definition of RIP abbreviation from the Oxford Advanced Learner's Dictionary

(also R.I.P. US English, British English)
BrE BrE//ˌɑːr aɪ ˈpiː//
 
; NAmE NAmE//ˌɑːr aɪ ˈpiː//
 
abbreviation rest in peace (often written on graves)
See the Oxford Advanced American Dictionary entry: RIP