gremlin

Short definition from Wiktionary

Noun

1. 1
   A contemptible person.
2. 2
   An imaginary creature reputed to be mischievously inclined, for example, to damage or dismantle machinery.
3. 3
   Any mysterious, unknown source of mischief or trouble, or the problem created thereby.
4. 4
   A young, inexperienced surfer or skateboarder, regarded as a nuisance.
5. 5
   A person regarded as similar to a fictional gremlin, in particular: mischievous, troublesome, short, or annoying.