‘Kakeru’ is a verb with a wide range of meanings. The relevant one here is the action of throwing something over something (like putting a blanket over someone). More narrowly, it is used for putting a liquid onto something. For instance, you would use it about throwing water on a fire, or putting sauce on top of food.

The ‘bu’ part is an intensifier indicating that the action is violent. ‘Tobasu’ means ‘to cause to fly’. ‘Buttobasu’ means ‘to send flying (violently)’. ‘Kowasu’ means ‘to break’. ‘Bukkowasu’ means ‘to break violently’. ‘Uchikomu’ means ‘to ram in’. ‘Buchikomu’ means ‘to ram in violently’.

Thus, ‘bukkakeru’ is an aggressive action of putting liquid over something. ‘Bukkake’ (the noun formed from this) is fairly transparent in its meaning — the concept of shooting liquid over the girl in an aggressive, humiliating manner.

Judging from the word in Japanese, it shouldn’t mean coming in a receptacle for the girl to guzzle. And I suspect it wouldn’t mean that for any Japanese. This extension of meaning could only be made by someone who doesn’t know Japanese.