On pre Solaris 2.1 SUNs it is in /usr/dict. ``This directory contains various files concerned with text handling commands such as spell" [p. 39]EF:UNIX. The file /usr/lib/dict/words contains over twenty-four thousand (24,000) words, in readable ASCII format.
The files hlista, hlistb, and hstop contain a list of ``American-only words, British-only words, and `stop-words' (words which the program may think acceptable, but which are not) for the spell command" [p. 39]EF:UNIX. These three files are hashed (i.e., we can't read them).