1580s, from Middle French frégate (1520s), from Italian fregata (Neapolitan fregate), which with many names for types of sea vessels is of unknown origin. It is common to the Mediterranean languages (Spanish, Portuguese, Catalan fragata). Originally a small, swift vessel; the word was applied to progressively larger types over the years.
[A] light nimble vessel built for speed; employed in particular for the gleaning of intelligence and the protection and assault of trade-routes. In battle the frigates took station on the disengaged side of the fleet, where they repeated signals, sped on messages, and succoured the distressed. [Sir Geoffrey Callender, "Sea Passages," 1943]
In the old sailing navy usually they carried guns on a raised quarter-deck and forecastle, hence frigate-built (1650s) of a vessel having the quarter-deck and forecastle raised above the main-deck.