Breeks is the Scots

Scots language

Scots is the Germanic language variety spoken in Lowland Scotland and parts of Ulster . It is sometimes called Lowland Scots to distinguish it from Scottish Gaelic, the Celtic language variety spoken in most of the western Highlands and in the Hebrides.Since there are no universally accepted...

 term for trousers

Trousers

Trousers are an item of clothing worn on the lower part of the body from the waist to the ankles, covering both legs separately...

, breeches and, as the Dictionary of the Scots Language (online) has it, also underpants.

From this it might be inferred that breeches and breeks relate to the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 references to the braccae

Braccae

Braccae is the Latin term for trousers, and in this context is today used to refer to a style of pants, made from wool. The Romans encountered this style of clothing among peoples whom they called Galli...

that were worn by the ancient Celt

Celt

The Celts were a diverse group of tribal societies in Iron Age and Roman-era Europe who spoke Celtic languages.The earliest archaeological culture commonly accepted as Celtic, or rather Proto-Celtic, was the central European Hallstatt culture , named for the rich grave finds in Hallstatt, Austria....

s, but the Oxford English Dictionary (also online) gives the etymology as "Common Germanic".

However, seeing as the Gaelic word for trousers is briogais, it is equally inferable that it simply derives via the Scottish term influencing Scottish English.

Outside Scotland the term breeks is often used to refer to breeches, a trouser similar to plus fours

Plus fours

Plus fours are breeches or trousers that extend 4 inches below the knee...

, especially when worn in Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 and engaging in field sports such as deer stalking

Deer stalking

Deer stalking is a British term for the stealthy pursuit of deer for sporting purposes, historically with dogs such as Scottish Deerhounds, or in modern times typically with a high powered rifle fitted with a telescopic sight to hunt them....

, and the activities of taking pheasant

Pheasant

Pheasants refer to some members of the Phasianinae subfamily of Phasianidae in the order Galliformes.Pheasants are characterised by strong sexual dimorphism, males being highly ornate with bright colours and adornments such as wattles and long tails. Males are usually larger than females and have...

, duck

Duck

Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

, partridge

Partridge

Partridges are birds in the pheasant family, Phasianidae. They are a non-migratory Old World group.These are medium-sized birds, intermediate between the larger pheasants and the smaller quails. Partridges are native to Europe, Asia, Africa, and the Middle East...

and other game birds. Whilst breeks are a neater, trimmer fit, plus twos are slightly wider with an extra 2 inches of material to fold over the knee, and plus fours a further 4 inches of material(and a wider, baggier fit).