The origin of the word 'shark' is unknown.

The earliest recorded use of the word in English goes back to the mid 1400s. It's thought likely that Europeans weren't familiar with larger varieties of shark before the beginning of voyages to the tropics (though the smaller relatives - dogfish - were very well known).

The etymology of the word shark is uncertain, the most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk, meaning 'villain, scoundrel' (cf card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour."

Source : Wikipedia

Another theory, proposed in the Oxford English Dictionary, was that it may have come from the Yucatec Maya word 'xok' (meaning shark) - but many etymologists now discount this.