The definition of boorish is rude and ill-mannered.
Chewing open mouthed while talking at the dinner table is an example of boorish.
like or characteristic of a boor; rude; awkward; ill-mannered
Resembling or characteristic of a boor; rude and clumsy in behavior.
(comparative more boorish, superlative most boorish)
boor (“peasant”) + -ish.