Fake cocktail/drink. The word exists purely as a way of cracking a joke at a barperson's expense whilst your mates are standing round, thus guaranteeing you kudos for the rest of the evening.
For best effect, order a few normal drinks, then finish off by requesting a matabooboo.
The word originates from Yogi Bear's sidekick. See example for full usage and relevance details.
Donna: "Who's next?"
Wes: "Yeah, can I have 3 pints of Stella, two double vodka red bulls, a sex on the beach and a matabooboo".
Donna: "A what?"
Wes: "A matabooboo"
Donna: "What's a matabooboo"
Wes: "Nothing Yogi, what's the matter with you?"
Donna: "... knob."