This is the older spelling of 'thoroughly.' In mod. editions of AV we find both forms used, 'thoroughly' in Ex 21:18; 2Ki 11:18, and 'throughly' elsewhere; but in the original edition of 1611 the spelling is 'throughly' everywhere. There was no distinction in earlier Eng. between 'through' and 'thorough,' 'throughly' and 'thoroughly.' In the first ed. of AV Ex 14:16 reads 'the children of Israel shall goe on dry ground thorow the mids of the Sea.'