(or is said to "have a hollow ring about it"), it's truth is suspect; the listener to it doesn't believe that the speaker is earnest in speaking it. (The derivation, I believe, has to do with making castings, specifically bells, in the early Industrial Age, where molding and casting techniques were not as refined as now. A bell or other casting that had a "hollow ring" to it was thought to have a flaw or imperfection that could not be seen, but could be heard in the vibrations of it when struck.)

In this particular case, I'm refering to your seeming offense at ben_tilly's use of the term, "Islam being spread by the sword", when you yourself have acknowledged that Islam was indeed foisted upon various Infidels with the implicit and/or explicit threat of bodily harm.