"May his nose soil with dust" means to suffer humiliation and disgrace. That is "may such person be humiliated and disgraced who hears my name and does not invoke Allah's blessings upon me." Those who merely kiss thumbs of their hands can also be the addressee of this saying because they do not supplicate Allah for the Prophet (PBUH) while what is in fact ordained is salutation, and not kissing of thumbs. The latter gesture has not been ordained in any Hadith. Some religious scholars regard the salutation obligatory while others hold that it desirable.