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fruit of the blackthorn, Old English slah (plural slan), from Proto-Germanic *slaikhwon (cf. Middle Dutch sleeu, Dutch slee, Old High German sleha, German Schlehe), from PIE *sleie- "blue, bluish, blue-black" (see livid).
The vowel has been influenced by that in the old plural form, which according to OED persisted into the 17c. Scottish slae preserves the older vowel. Sloe-eyed is attested from 1804; sloe gin first recorded 1878.
(Prunus spinosa), spiny shrub, of the rose family (Rosaceae), native to Europe but cultivated in other regions. The name is also applied to Crataegus calpodendron (or C. tomentosa), commonly called pear haw, another shrub or small tree of the rose family. P. spinosa usually grows less than 3.6 metres (12 feet) tall and has numerous, small leaves. Its dense growth makes it suitable for hedges. The white flowers, about 2 centimetres (0.8 inch) in diameter, appear before the leaves. The bluish-black, tart-flavoured fruit is about 2 cm in diameter and is used to flavour sloe gin.