Penthland Firth is the name of the treacherous strait between Caithness and the Orkney Islands. It was- and is - a very dangerous area for shipping because it has one of the most powerful tidal currents in the world. This can create tidal swell waves several metres in height. In the eastern part of the strait, currents up to 16 knots have been measured.

Areas where the current and the tidal swell are particularly pronounced have been given names such as The Merry Men of May and The Swilkie. The latter also sometimes creates a maelstrom or whirlpool. The name Swilkie has Nordic origins meaning something like “the swallower”. According to a legend, said to date back to Viking times and also known from the Younger Edda, the maelstrom is created by a sea witch rotating the quern that grinds salt for the sea – hence the word maelstrom (mael = grind; strom = current). The Swilkie originates from a point on the small island of Stroma, whose name also has Nordic origins: “Strøm Ø” (Current Island). The place names clearly speak of the dangers of the strait!

From the Viking Museum, Denmark
www.vikingeskibsmuseet.dk/en/professions/education/the-viking-age-geography/the-vikings-in-the-west/scotland/pentland-firth

The name, Pentland Firth, is presumed to be a corruption of the Old Norse "Petlandsfjörð", meaning "the fjord of Pictland"

Prior to the Norse occupation of Orkney the strait was known as the "Sea of Orcs" – a reference to the Pictish tribe who inhabited Orkney. Such was their marine prowess that there are also instances of this name referring to the sea lanes of the entire west coast of Scotland down to Kintyre. One version of the 9th-century Historia Brittonum states that "the Britons originally filled the whole island with their peoples from the English Channel to the Sea of Orcs.

From Wikipedia