It is an adjective.
Meaning: Impolite or abrupt. When any process is interrupted rudely in the middle without the due formalities we will call that as unceremonious.
unceremoniously - adverb.
unceremoniousness - noun.
Synonyms : abrupt, sudden, hasty, hurried, summary, perfunctory, undignified, rude, impolite, uncivil.
Antonyms : polite,formal,ceremonious
1)The politician speech came to an unceremonious halt due to a power breakdown.
2)The presentation was unceremoniously interrupted by the ringing of mobile phone.