Origin: ME (t)riding < OE -thrithing, a third part (only in L contexts) < ON thrithjungr < thrithi, third: initial t was lost to the preceding sound in compounds formed with North-, East-, and West-
See riding in American Heritage Dictionary 4
Origin: Middle English
Origin: , alteration of trithing
Origin: , from Old English *thrithing
Origin: , from Old Norse thridhjungr, third part
Origin: , from thridhi, third; see trei- in Indo-European roots.
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