[common; from mainstream slang meaning ‘vomit’]
1. interj. Term of disgust. This is the closest hackish equivalent of the Valspeak “gag me with a spoon”. (Like, euwww!) See bletch.
2. vi. To say “Barf!” or emit some similar expression of disgust. “I showed him my latest hack and he barfed” means only that he complained about it, not that he literally vomited.
3. vi. To fail to work because of unacceptable input, perhaps with a suitable error message, perhaps not. Examples: “The division operation barfs if you try to divide by 0.” (That is, the division operation checks for an attempt to divide by zero, and if one is encountered it causes the operation to fail in some unspecified, but generally obvious, manner.) “The text editor barfs if you try to read in a new file before writing out the old one.”
See choke. In Commonwealth Hackish, barf is generally replaced by ‘puke’ or ‘vom’. barf is sometimes also used as a metasyntactic variable, like foo or bar.