Synonyms for Loudmouth:

adj.

boastful (adjective)
know-it-all.

n.

horror, yob, public enemy number one, vermin, (the) scum of the earth, creep, public nuisance, know-all. blabbermouth (noun)
windbag, gasbag.
braggart (noun)
bluffer, swaggerer, egoist, boaster, know-it-all, braggart, narcissist, gasbag, swellhead, windbag, blusterer, blower.

Other synonyms:

creep, public nuisance, yob. vermin. horror.