This word is an obvious insult and originates in some "olde English" insults. Those insults are:
1. Donkey-sucker - also can be used as "you suck the donkey"
2. Freshman - To be concise, they suck the donkey
3. Flaker - refer to the word flak
Yeah, well, shut up you Donkey-Sucking-Freshman-Flaker!