Definition from Wiktionary, the free dictionary
Etymology 1 
Alteration of girth (“belt, circumference, brace”)
girt (plural girts)
- A horizontal structural member of post and beam architecture, typically attached to bridge two or more vertical members such as corner posts.
Related terms 
Etymology 2 
From Middle English girten (“gird, encircle”)
Etymology 3 
- simple past tense and past participle of gird
girt (not comparable)
- (nautical) Bound by a cable; used of a vessel so moored by two anchors that she swings against one of the cables by force of the current or tide.