Obviously a compound word, this is used as a question to challenge an opponent to a duel or battle of some kind.
One is asking another in anger if they would care to step to them or rather if they would like to take it outside.
This should be spoken as quickly and as clearly as possible for the proper effect. If not, perhaps because one is drunk, it might come out as the word fishy.
A corruption of the common term: bastard
It was editted down to as(s)tard a rather fun word form still in usage today- but given the continued laziness of the human tongue and the need to be discreet among the easily offended this was futher shortened down. The 'd' became a 'g' and now the word could be passed for either a fatherless ass of a person or a Klingon pet, kind of like a dog.