J.J. Gould contends that dictionaries aren’t enough:
Dude may be the most Mandarin Chinese word in American English. In Mandarin, depending on how I intone the single syllable ma, I could be saying “mother” (mā), or I could be saying something as radically distinct as “horse” (mă). Dude has a comparable quality. Just think of the last time you did something awesome in the presence of a friend who affirmed your awesomeness with the exclamation Duuude! Or the last time you said something objectionable to someone who began setting you straight with a firm and sober Dude. There may not be any obvious difference in denotation between these cases, but the difference in connotation is, you’ll appreciate from experience, pretty major.
Update from a reader:
This may be the first instance in the history of the Internet that someone has email-forwarded the work of Rob Schneider, but he unpacked the varied meanings of dude back in the late-’80s:
And there’s this classic beer commercial: