In his De Bello Gallico Caesar give a short and enigmatic description of the inhabitants of Britain. Enigmatic, because he assigns to them the custom of polyandry. Polyandry – the sharing of a woman between various men – is rarely found and is best known from Himalayan tribes. Caesar also says that they were clad in skins, but this is definitely not true as wool and linen were in use too. It could be that he wanted them to depict as primitive as possible. Still, it could be that Caesar was somehow right about polyandry, but that can hardly have been a practice of the whole population, as there would have been an enormous amount of unmarried women. Also, granted that it did exist, the number of ten to twelve men for each woman seems absurd high.
It is also possible that this custom did exist, but not amongst the Celtic tribes, but with pre-Celtic tribes. In this case the Picts come to mind. Their origin is unclear and though some claim they are a Celtic tribe, this is far from sure. Their name comes from Roman pictus as they painted or tattooed their bodies, which would be in line with what Caesar says is this passage too. How the Picts called themselves is unknown. So it is possible that Caesar is conflating Celtic and Pictish customs.
Some commentators have seen this passage as a proof for the high esteem women were held in amongst the Celts, as if polyandry mirrors polygamy, but I doubt that, though women were certainly respected and could even become leaders, be it in exceptional circumstances, as Boudicca proves. The wiki link below clams on this single reference that this practice was common amongst the Celts, but Caesar says nothing about this practice in Gaul. So be careful with claims on Wikipedia: check and double check! I am leaving aside all those claims on internet that Celtic women had a great sexual freedom as if they were happy hippies from the sixties. There is always a great danger of reading into texts once own ideology…
Caesar, De Bello Gallico, V.14
Ex his omnibus longe sunt humanissimi qui Cantium incolunt, quae regio est maritima omnis, neque multum a Gallica differunt consuetudine. Interiores plerique frumenta non serunt, sed lacte et carne vivunt pellibusque sunt vestiti. Omnes vero se Britanni vitro inficiunt, quod caeruleum efficit colorem, atque hoc horridiores sunt in pugna aspectu; capilloque sunt promisso atque omni parte corporis rasa praeter caput et labrum superius. Uxores habent deni duodenique inter se communes et maxime fratres cum fratribus parentesque cum liberis; sed qui sunt ex his nati, eorum habentur liberi, quo primum virgo quaeque deducta est.
Ex his omnibus: i.e. of all the inhabitants of Britain.
maritima omnis: actually only half of Kent is surrounded by the sea
interiores: those living away from the sea
frumenta: kinds of wheat
sero sevi satum: to sow (from the same Indo-European root as English `seed’.)
vitrum: woad (Isatis tinctoria, a plant used for its blue colour.)
inficio infeci infectum: to paint
aspectu: ablativius limitationis with horridiores.
capillo promisso: loose hanging hair (ablatives descriptionis)
rado rasi rasum: to shave
deni: ten each
parentesque cum liberis: of course only fathers and sons
habentur: are counted as
quo = ad quem
deduco deduxi deductum: to marry (A bride was led in procession to the house of her bridegroom.)