Definition of "thro"

thro¹

preposition
1. (archaic) through
  Quotations
  He the Red Sea rebuk'd also,That it updrying fled:As thro a desert dry to go,Them thro the deeps He led.
  1851, Montagu, The Psalms, in a New Version, Fitted to the Tunes Used in Churches: Psalm CVI

thro²

adjective
comparative more thro, superlative most thro
1. (obsolete) Eager; earnest; vehement.
2. (obsolete) Bold.