Varmint
1. Vermin. An animal of a noxious or objectionable kind.
  
2. An objectionable or troublesome person; a mischievous boy or child.
3. Knowing, clever, cunning.
Also, 4. A sporting amateur with the knowledge or skill of a professional.

The OED gives the derivation for meanings 1 & 2 as a variant of vermin with excrescent. Although the first recorded use is in 1539 the word is said to be rare before about 1825. There is apparently no obvious connection between the word as used in meanings 1 & 2 and that of meanings 3 & 4, which I find hard to believe.