Used chiefly in contexts involving horses or animals to mean ‘to command to stop’ and, by extension, as an intransitive verb meaning ‘to stop or slow down’.
Definitions
To stop or slow down in response to the command “whoa.” (intransitive)
The mare whoaed suddenly at the edge of the trail.
To command (a horse, team, or vehicle) to stop or slow down by calling “whoa.” (transitive)
She whoaed the sled dogs before they reached the river.