The story
A collateral form of fortnight with the dialectal change of /xt/ to /θ/ (probably via an intermediate /θt/); compare Doric Scots dother (“daughter”), mith (“might”), and obsolete noth (“nothing”). Note that drouth (“drought”) is not an example of this change; it instead retains the dental fricative of Old English drugaþ.
Etymology from Wiktionary, CC BY-SA 4.0.