This is one of a family of words coming from Latin that use the suffix -aster, which is a nouns suffix that expresses incomplete resemblance, and so are usually pejorative. The most common one these days is poetaster (a bad poet), but others include grammaticaster (a petty grammarian, esp. one who is wrong), philosophaster ("a pretender to philosophy"), and theologaster (a petty or contemptable theologian). This one entered English (probably via either Italian or French intermediation) around 1600. Nowadays its use is mainly literary. Usage example:
Opium is a double-edged sword, a divine gift in the hands of a master, a poison in those of a mere routinist—a medicaster—a demi-physician.
(Ooo -- I'll have to remember that demi-physician one.)