smart aleck

Short definition from Wiktionary

Noun

1. 1
   One who is given to obnoxious or insolent humor.
2. 2
   One who is pretentious about their own cleverness or knowledge.
3. 3
   One who is obnoxiously self-assured; a show off.
4. 4
   A well-dressed person.

Adjective

1. 1
   Characteristic of a smart aleck.