1. Generally used as an indistinct insult to describe the attributes in behaviour of an individual that is inoffensive but very annoying.
2. Can also be used as a term of endearment for an inoffensive friend or associate; often a looser at every enterprise undertaken.
In either case, the person will act like a tit
, in reference to behaviour or association with a cock
. Not to be confused with a cocksucker
who is a wanker