From The Jargon File (version 4.4.7, 29 Dec 2003) :
prettyprint /prit'ee.print/, v. (alt.: pretty-print) 1. To generate `pretty' human-readable output from a hairy internal representation; esp. used for the process of grinding (sense 1) program code, and most esp. for LISP code. 2. To format in some particularly slick and nontrivial way.