thorough-

• through- (used before a vowel)

A variant of through-, common in 16th-19th centuries, particularly where there is a perceived connection with thorough “thorough-”, in OED Online ⁠, Oxford, Oxfordshire: Oxford University Press, launched 2000..

thorough-

1. (non-productive) through, passing via

   thoroughfare, thoroughgo, thoroughlane

2. (non-productive) fully, completely; purely

   thoroughgoingly, thoroughbreed, thoroughpaced

• (via): dia-
• (fully): per-

• thoroughbrace
• thoroughpin