When someone has sexual intercourse with a rather large female or male (or more appropriately a feWHALE or maleWHALE).
To BE moby-dicked would occur when said feWHALE or maleWHALE engages in intercourse with a more slender female or male.
These phrases work best when there is a noticeable discrepancy between the two mammals engaging in intercourse.
The etymology of the word can be traced to Herman Melville's novel "Moby Dick," alternately titled "The Whale." While I don't think "moby-dicking" occurs in the novel, some sick bastards have imagination.
"-How was Vegas brah?
-Man your boii got too drunk and was moby-dicking all night. Sometimes your boii gets desperate, nah mean?"
"-How was Vegas homegurrl
-Met a victim. Got moby-dicked ;)"