If you know what a word means but want to know why it has that meaning - Online Etymology Dictionary. (There's also a link to Dictionary.com if you need a definition also.)
blog
1998, short for weblog (which is attested from 1994, though not in the sense 'online journal'), from (World Wide) Web + log. Joe Bloggs (c.1969) was British slang for "any hypothetical person" (cf. U.S. equivalent Joe Blow).