According to Wikipedia:
grep is a command line text search utility originally written for Unix. The name is taken from the first letters in global / regular expression / print, a series of instructions in text editors such as ed.1 A backronym of the unusual name also exists in the form of Generalized Regular Expression Parser. The grep command searches files or standard input globally for lines matching a given regular expression, and prints them to the program's standard output.
Grep was created by Ken Thompson as a standalone application adapted from the regular expression parser he had written for the ed editor (which he also created). The name grep comes from the ed editor command it simulated, g/re/p (global regular expression print). Its official date of creation is given as March 3, 1973 in the Manual for Unix Version 4.