A numnick is an integer encoded in a format similar to Base64. This encoding is used in IRCu Protocol 10 (P10) inter-server communications. The encoding uses characters from the set of upper and lower case English letters (a-z), digits zero (0) through nine (9), and the open () and close (
) square brackets.
HOWEVER, I use it as an Anglicised version of the Yiddish Nudnick, meaning a pest, a bore, sometimes used as a term of endearment. If you read the official definition above it translates to an anorack or nurd.
When someone is boring you with their exploits as a train-spotter or fisherman or their grandchildren, they are a numnick.
When your child/partner does something silly or endearing, with great affection you say "You are a numnick, but I love you".