Apparently the word "triangle" was borrowed into English in the late 1300s. Triangles are a very common shape in everyday life, and there were certainly English-speaking craftsmen and artists before the 1300s who would have needed a word for them.
What did English call triangles before they borrowed the word "triangle"?
Gore was used in OE to convey the idea of a triangle:
- "triangular piece of ground," Old English gara "corner, point of land, cape, promontory," from Proto-Germanic *gaizon- (cognates: Old Frisian gare "a gore of cloth; a garment," Dutch geer, German gehre "a wedge, a gore"), from PIE *ghaiso- "a stick, spear" (see gar). The connecting sense is "triangularity." Hence also the senses "front of a skirt" (mid-13c.), and "triangular piece of cloth" (early 14c.).
(Etymonline)
In Old English, the adjective þryscyte 'triangular' (Icelandic þriskeyta n. 'triangle') was used to define triangular shapes. Additionally, gar~gara suggests triangularity but it was only used for land masses and it is not always translated as a triangular land mass.
Here is a dictionary entry for þryscyte from An Anglo-Saxon Dictionary by Bosworth and Toller (1898):
þri-scíte; adj. Triangular, three-cornered :-- Ispania land is þryscýte Hispania trigona est, Ors. 1, 1; Swt. 24, 1. Sicilia is ðryscýte Sicilia tria habet promontoria, Swt. 28, 2. On ðone þryscýtan crundel, Cod. Dip. Kmbl. v. 374, 26. [Cf. Icel. þrí-skeyta a triangle.]
Here is an excerpt from the book Grammaticalization at Work: Studies of Long-term Developments in English (edited by Matti Rissanen, Merja Kytö, Kirsi Heikkonen) where you can see the translation of both Old English words:
Note: You can see the original text in archive.org also.
You can read much more details about geometric terms in Old English in the book Folk-taxonomies in Early English (By Earl R. Anderson) which also includes the following explanations:
Although Anglo-Saxon and medieval societies were "carpentered worlds", Old English had basic terms only for curvilinearity (hring and beag), and until the fourteenth century Middle English had only circul~circle and ring.
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Like other Indo-European languages, Old English has no taxonym for 'geometric shape'; OE hiw (Gothic hiwi) usually means 'appearance'.
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Because of the paucity of geometric terms in Old English, buildings in poetry may be described in terms of their height and magnitude, but rarely in terms of shape.
