[from the qed/ed editor idiom g/re/p, where re stands for a regular expression, to Globally search for the Regular Expression and Print the lines containing matches to it, via Unix grep(1)] To rapidly scan a file or set of files looking for a particular string or pattern (when browsing through a large set of files, one may speak of grepping around). By extension, to look for something by pattern. “Grep the bulletin board for the system backup schedule, would you?” See also vgrep.
[It has been alleged that the source is from the title of a paper “A General Regular Expression Parser”, but dmr confirms the g/re/p etymology --ESR]