: an inferior poet
It turns out the suffix -aster means "second-rate." Hence words like POETASTER, which I initially read as "poe taster." (Huh?)
I think this word and others like it are pretty useless. It's just a means for snobs to verbally insult people via throwing a Latin suffix onto the end of a word and hence sounding more learned. What a joke. If I ever actually heard someone saying the word POETASTER I would punch him in the face.
I did, however, discover some other -asters that were interesting:
mathematicaster - minor or inferior mathematician
logicaster - a petty logician
and my favorite...
usageaster - self-appointed and conservative language usage expert.
For a more comprehensive list, look at THE PHRONTISTERY, aka "a thinking place"—another website devoted to "spreading the joy of the English language." Most of the definitions on the list imply the idea of a petty or mediocre version of someone more talented, serious, or true. Phrontistery's web-m(aster) notes that these words are little used today.
Why? Because one could bypass all the -asters and simply say "you suck."