Galligaskins were wide, very loose breeches. They were a fashion of the sixteenth and seventeenth centuries, one that eventually disappeared, as did the active use of the word, which survives in historical contexts or as a humorous word for nether garments.
It’s an odd-looking word, well fitted to the epithet weird. It came about through another of those cloth-eared Englishmen’s attempts at getting their minds around a foreign term. They knew it was French in immediate origin, gargesque, and they knew the garments were often worn by sailors, so they assumed that the first part was galley, either from the oared ship, or from the cooking area on board ship. Similar items were known at about the same time as gally-slops or gally-breeches, so that would easily account for the conversion of the first element of the French word into something more English-sounding. (The first of these was often abbreviated to slops, a similar item; the material for them was kept on board ship in the slop-chest, though sailors’ working garments called slops, at least of a later period, were loose trousers rather than breeches.)
Some dictionaries of the eighteenth and nineteenth centuries suggested that the word was from Gallic Gascons, the inhabitants of Gascony. They were looking too far west — the French word was taken from the Italian grechesca, something Greek, because the fashion for loose breeches was originally from that country. Around the years 1580-1620 similar garments were called Venetians, because a comparable fashion had been imported from Venice.
Galligaskins made a relatively late appearance in Sir Nigel, an historical novel by Arthur Conan Doyle, published in 1906: “It was a wretched, rutted mule-track running through thick forests with occasional clearings in which lay the small Kentish villages, where rude shock-headed peasants with smocks and galligaskins stared with bold, greedy eyes at the travellers.” This is probably a different sense of the word, since the English Dialect Dictionary says that it was used in Kent and other counties for work leggings, which it described as “rough leather overalls, worn by thatchers, hedgers, and labourers. They are usually home-made from dried raw skin, and are fastened to the front only of the leg and thigh.”