On the Pirates of the Caribbean ride at Disneyland.

Your pirate lesson for the day:

To a pirate or any member of the nautical community, “avast” means to stop or desist.
The term avast has been integrated so thoroughly into piratespeak that few individuals
are aware of the actual meaning of the phrase. It is commonly misused in place of “ahoy,”
which is a nautical greeting. Individuals who would like to talk like a pirate accurately
may want to invest in a nautical dictionary or do some research on traditional sailing terms.

The word "avast" was first documented in 1681, and likely originated from a Dutch sailing
term, houd vast, which means to hold fast. The term could refer to military action or the
necessity to hold firmly onto ropes and lines aboard a ship. Avast has been widely used in
the maritime community ever since as an interjection much like stop or halt.
: http://www.wisegeek.com/to-a-pirate-what-does-avast-mean.htm