I don't think it was meant to fill a semantic hole. It was a silly conversation amongst tired language teachers and the result was us cackling like loons at the word horrend and it's variant, horrended.

I think it started when my friend was musing about how it's 'horrify' and 'horrified' but then 'horrendous' instead of something like 'horrifous'. Or, if -ify is a verbal suffix, shouldn't it be 'horrous'? Was the root horr- or horrif-? If the root is horr- then where did the -endous come from instead of just the -ous? Maybe the root is horrend?
And so on.